Add your turrets to damage/attack speed upgrades (through tech) — based on gun turret upgrade present in tech.
See:
1) https://wiki.factorio.com/Physical_projectile_damage_(research)
2) https://wiki.factorio.com/Weapon_shooting_speed_(research)

Why:
if we play death worlds with strong enemies like Swarmageddon + Rampant + Armored biters + etc. — your turrets become useless at evolution factor>=50%
With upgrades >= lvl5 — I prefer to use standard turrets because of more damage to biters.

maybe everything works for you and so?
Tested with mods: Swarmageddon + Rampant + Armored biters + AAI pack of mods + SE pack of mods + This mod.

PS. your laser/beam/scattergun turrets works fine with upgrades, but not with your gun/cannon turrets

EDIT: tested with vanilla base game + this mod — your gun/cannon turrets are ignored in bullet/cannon upgrades for damage and attack speed.
Please, fix it

Adding +Turret Damage to ballistic turrets makes them extremely overpowered. I've taken away those bonuses long time ago, because they made defenses trivial ( like killing a group of behemoth biters with a single explosive rocket or shell before repeatables ).

What is your defence setup, or playstyle in general? How far you are in the tech tree? I see you've mentioned the beam turrets, which are post-rocket launch - in that case you should have pretty much all turrets unlocked. Are you using area of effect or slowing ammo for rocket/cannon turrets? Are you using efficiency modules to reduce pollution? Did you try nukes in rocket turrets?

You're playing a mod set that is not only incredibly punishing, but also very slow to play, with SE being the core tech mod. For comparison, I have a late game Krastorio 2 / Rampant / Armored Biters save at 95% Evo, and bugs can't even make it to the walls. It looks pretty much the same to the far right screenshot on the main page - Scatterguns, then Heavy MGs, then Dual Cannon / Rocket in 2:1 ratio, with uranium shells and explosive rocket packs as ammo.

This mod was made with variety in mind - not power. If your complaint is that turrets are not overpowered enough to match your very, VERY extreme difficulty you've chosen to play, then it's on you to find solutions - either try adjusting your playstyle and find defence setups that work (which should be doable, considering the amount of area of effect weaponry available in the mod), switch to another turret mod that is made specifically for the extreme difficulty you want (like Rampant Arsenal), or take away a mod or two (I think it's Swarmageddon specifically, with its recursive biter spawns on death, that's the source of issues).

oh, thanks.
I play with Rampant (fixed) + Swarmageddon + AAI pack + SE pack + Modular Turrets and with the standard Death World settings.
With SE we'll get 40-50% evo when you want get first land oil — before explosive turrets, and uranium/slowing/area ammo. But, yeah, I can fix this via settings or mods (additional powerful bullets up to uranium) ...

Anyway, it's strange why player get so big upgrades for standard gun turrets and scattergun turrets, and do not get upgrades for modular ammo turrets, but modular laser/beam turrets have all upgrades.

P.S. I did some math and I think I got your idea:
– For turrets with bullets, we do not get bonuses because your turrets immediately have a permanent bonus +100%, and then with the transition to gatling turrets — a much higher attack speed. I mean, you are suggesting that we completely abandon standard and heavy MG turrets as soon as we get to gatling turrets. At least until we pump the damage upgrades through very many levels (infinity QoL research)
– For cannon turrets, which have no analogue in the vanilla game, you only rely on shells with explosions — they are already powerful.

Oh, ok, so you're before oil and already at big biters, that makes it tricky... Hmm, The only suggestion I have, is to rush flamethrower turrets, if they are still red/green/military research. For defense until then, densely packed scatterguns followed by 2 - 4 layers of heavy MG turrets. If you're fighting manually as well, grenades are great at clearing small stuff.

Reason vanilla gun turrets have both +bullet and +turret damage upgrades, is they actually need them lategame to be of any use - behemoth spitters have a range of 16 VS 18 on gun turrets, they will always exchange fire - so turrets have to out-DPS behemoth spitters by quite a lot to chew through 1500 health each spitter has (with only a mealsy 400 HP and no resistances on the turret, so it dies easily to spitters).

Same reasoning goes to Scattergun Turrets - they are bulky, but lack protection of walls from melee enemies and DPS without the turret damage upgrade against behemoth biters is too low to make a sizable dent in them (with maxed non-repeatable research, when turret damage bonus is taken away, gives estimated 1.5k DPS with uranium ammo, assuming all pellets hit, VS 4500 HP and 12/10% resistance on each behemoth biter) - turret will get like 2 shots off before it gets swarmed and takes a pounding.

Other modded turrets don't have this problem - they are bulky (1500 HP and resistances), and long ranges means they clear biters without being at risk of taking serious damage themselves. That's why everything else only gets one damage bonus (either +ammo damage for ballistics, or +turret damage for energy) - they have much bigger time window to apply the damage they have and not die themselves.

As for turret composition, outside of Light Cannon -> Dual Cannon, there are no direct upgrades, each turret is a tradeoff between various values. Replacing all HMGs with gatlings is a bad idea - if you do this, you lose the overlap, as gatlings have narrow firing cone. I'd instead recommend either swapping HMGs for gatlings in hotspots (replace 2-3 in places where gatlings will shoot forward anyway, and leave HMGs to the left/right for overlapping firing cones), or replace entire first row, and move all HMG's behind gatlings to create a second row. In general, my modded turrets have the best result when you mix them to cover each other's - they are complementary, not competitive with each over.

thanks

I have thoroughly read this thread, and, whilst I feel the dev's points, I still totally agree with @saaadel.
Frankly, the fact that these turrets dont have 360 angle shoot is already a huge balance effect vs the vanilla turrets, as you cannot plant 20 of them in a second, to push a biter's base.
I love this mod, its concept and execution, but I believe that the turrets should scale 100% like the vanilla turrets. So what if they are stronger than vanilla? It's cool, and u need to piece your turrets together. Honestly, if I could I would just use your turrets and get rid of the vanilla ones.

Also, @saadel's set up isn't "incredibly punishing". For people playing Factorio for years, it's natural that one would seek to gradually test the limits of the game's difficulty. Ironically, one would think that this mod would cater to precisely those people, those who are interested in specialized, and a more complex system of turrets.
Anyway, I look forward to you putting the upgrades back on, or a setting of some sort XD

Oh, you totally can do turret creep with these turrets - it just looks way different. And I say it from position of disliking the tactic.

I think the scaling problems saaadel had was based on the specific modset - mainly SE. That mod significantly slows down tech progression, so stuff like area of effect ammunition for cannon turrets, which you'd normally get around the point it becoming relevant, is heavily pushed back and unavailable, while biters scale much faster due to amount of pollution produced to sustain K2+SE requirements, not to mention quadratically more enemies to kill due to Swarmageddon spawning lower tier enemies from higher tier ones - there is a point where single target damage is simply unable to deal with the swarm. Would extra 40% damage from 3 first +damage techs solve the issue? It would help a bit, sure, but it would only delay a problem, not solve it. I played vanilla, deathworld vanilla, rampant vanilla, K2, IR3 and currently K2 deathworld, and felt balancing was reasonable for all of them.

I just feel, if you're going to push difficulty THAT FAR, maybe a different turret mod would be a solution. I created this mod for vanilla+ progression, and I'd agree, some elements will fall apart when used significantly outside that progression style, which SE certainly counts toward.

That said, implementing options is always a good thing, so I'm all in for optional setting to grant multiplicative turret damage bonuses to non-energy turrets back. Just not right now - current heat wave makes my laptop melt and cry for help, so as soon as temps go back to reasonable levels its getting implemented.