Would be nice to have at least one slot for a custom weapon available from the start, so we can use other mods weapons/ammo.

Unfortunately custom weapon slots are not possible to have in vehicles. If you have a particular mod in mind, you can suggest which weapons should be used from it and which tiers they should be unlocked at.

Oh well, didn't know that. I think doing changes to every single weapon mod would be a good idea.

Anyway, I usually play full bob/angels + youki + biters mods, like explosive biters, Revamp, etc. So the bitters get quite strong before you have a good production up and running or even get some good way to get a defense up, so I usually go to youki turrets/weapons that are quite strong and aren't locked behind anything other than been able to make them.

The YI-HX5 Minigun for example, to be fare in normal game it would be need to be at least mk3+, but for a harder game, mk2 would make you safer.
With angels the starting point for the youki you have to have at least steel running so it isn't super fast considering all the sciences that were introduced by angels