Please put all your suggestions here for Vanilla Underhaul.

I did a scan over the vanilla tech tree and these are my notes (as of version 0.6.5):

Rename "Fast inserter" to "Inserter" or "Electric inserter"
Rename "Assembling machine 2" to "Assembling machine"
Rename "Fast transport belt" + siblings to "Transport belt"
Getting rid of Defender robots, OR make them available at Red science
Maybe condense Power armor Mk2 into Power armor
Maybe replace some equipment modules with their Mk2 version
Make "Nightvision" equipment 1x1
Possibly increase Uranium-235 chance to 1%
Possibly simplify Spidertron recipe (e.g. remove fish)
Possibly make "Recycling" technology cheaper (when Quality mod is active)
"Modules" techs need to include Quality modules (when Quality mod is active)
Alter blueprint-related Shortcut buttons so you have them unlocked from the beginning of the game.
Hide "Create discharge defense remote" shortcut button.
Perhaps merge Heavy armor and Heavy armor+
Perhaps merge Concrete with Refined concrete (so Concrete has the higher bonus)
Perhaps respawn the player with Submachine gun after they research it.

New Oil refinery recipes from "Advanced oil processing" (so the player isn't forced into understanding cracking/circuits)
- "Heavy oil extraction". 50 Water + 100 Crude -> 25 Heavy oil
- "Light oil extraction". 50 Water + 100 Crude -> 45 Light oil
Have a description on these recipes to indicate that they are wasteful!

If you want to exclude Quality as a mechanic, since Space Age requires the quality mod to be active, look into the "No More Quality" mod for how you could hide it.

Technologies that are currently not "hidden" but using "enabled = false":

Hide "Automation 3" technology
Hide "Lubricant" technology
Hide "Logistics 3" technology
Technology for "Bulk inserter" is redundant and doesn't do anything
Hide "Discharge defence" technology
Hide "Effect transmission" technology (i.e. Beacons)
Hide "Atomic bomb" technology

Hide "Utility science pack" technology
Remove Utility science pack requirement from:
- Braking force 6 + 7
- Lab research speed 6
- Weapon shooting speed 6
- Worker robot cargo size 3
- Epic + Legendary quality (when Quality mod is active)

The Tech issues have been resolved!
For the quality changes I have not tried the mod with only quality installed so I have yet to add compatibility.

And with the oil refining idea, I really don't know. I don't want to remove mechanics completely if possible and I wonder whether this would be the correct choice. I am leaning towards it being a good idea though.

For oil, you could make it so that advanced refining produces Heavy oil, and then only allow cracking heavy -> Light. Entirely linear, and no circuits needed.

You might want to tweak smelting times. Both furnaces should have a speed of 1, and smelting time should be 1. This results in a 30 furnace requirement either way, I think. Or smelting time of 1.6, for 48, I suppose, if consistency to vanilla and a composite number is preferable, which is fair.

Remove pistol from the game/replace entirely with SMG? Also remove shotgun and place Combat shotgun in its place. No need to have 2 versions of a weapon here, right?

Engine unit:Electric engine unit isn't a nice 1:1, or even 1:2, which kinda sucks. Suggest electric engine drop to 6 seconds, or engine increase to 10 seconds (and then roboframe drop to 10 as well)

Swap the fast inserter and long handed inserter positions. Yea, thats vanilla, but it just looks wrong having long then short instead of the other way around. Optionally, also make red inserters have fast inserter speed. Though that might be out of scope.

Concrete could use iron plates instead of ore, I suppose.

The assembly machine 2 probably needs renamed to Assembly Machine.

Removing the cracking recipe for light to petroleum would make the cracking mechanic harder not easier. (I had misunderstood the proposal)

Tweeking smelting times is a good idea, I will look at the ratios.

For the Engines I would need to maintain the ratio for the sciences so I will likely have to change the ratio from the side of the electric engine.

Long inserters being half as fast as short ones is purposeful balance decision.

When it comes to military and personal equipment items I am still deciding what to do as, I don't want to remove military progression for people that play with biters.

On cracking. How so? You have 2 refinery recipes. Petroleum and heavy oil. Then one cracking recipe. Heavy to light. Now there is exactly one path for each of the 3 oil types.

I suppose you might want to increase the petroleum recipe yield to loosely match the full advanced and cracking down to Petro yield, but that simplifies the entire mess massively.