Did some testing how to eliminate AI derelict colony ship problems in Huge galaxies (254 systems). Following changes were enough that the AI never had colony ships that run out of supply.
1. Added 1000 Supply storage to colony ship modules.
2. Changed engines so that they take supplies as 10,9,8 and made it so that AI uses type III engines (8 supply usage) most of the time...
3. Changed colony module size to 180kt so that AI could add Solar collector component to its colony ship designs. Also made sure that the AI researches stellar harnessing III pretty early in the game, its not expensive: 50k+75k=125k.
With these changes i would say that there are practically almost no limits to colony ships range. Standard colony ship with type III ion engines consumes 200 supplies per turn and the solar collector III collects 150 supplies per turn, so the colony ship uses only 50 supplies per turn now. It has an average operating time of 70 turns now and could move 350 sectors. That guarantees that the colony ship can now travel through 31 systems on average... i think that is enough even in the largest galaxies
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Also i made it so that each AI carrier had 3 solar collectors in its designs. As the AI currently likes to include carriers in its fleets these fleets had much more range... i.e AI fleets could attack several different planets/systems until they had to return back to resupply (note that this has almost no effect on AI carriers since they have only 2 less fighter bays). I've also been thinking to add solar collectors on attack ships, and if you play on Huge galaxies (like 200+ systems) i think it would be wise to add 1 or 2 solar collectors to each AI attack ship design too (depending on the size of these attack ships ofcourse).