Early game you need a large fleet to keep the computer players off your back. At about midgame suddenly the pressure eases dramatically . My best guess for this is that the computer players are using all their metal to build starbases. Perhaps the AI should reserve a minimum amount for ship production each turn? Perhaps 1/2 there metal output?

Interesting theory

The AI does seems to be buildinga lot of starbases as soon as mid-game hits
There has been an update a coupld of moths ago which has caused them building more starbases - as previously there seems to have been an issue with their priorisation.

Maybe if the priority of building starbases is lowered a bit, that would resolve the issue?
I would be hesitant to give a flat out bonus to the AI, as the balance is morphed a lot by such mechanisms (explorer ship spam could be made worse for example)

I didn't mean a bonus, but a reserve. IE if I have only 1000 metal, I reserve 500 for shipbuilding, as opposed to dumping it all on starbases.

The AI uses special rules for dealing with metal. Which is to say, if an AI faction finds itself with a metal deficit, it has triggers to take actions to remedy the situation, but it doesn't actually suffer the normal production penalties for being short on metal (it can convert labor to metal at a favorable rate). So I don't think it's a lack of metal that is causing the behavior that you're seeing.

I don't know for sure what the issue is, since this is not really my department, but I would guess that it has more to do with the AI losing large numbers of ships in battles with pirates and other factions, and not having a high enough priority on replacing them.

I modded fleet bases to reduce base metal cost to .8 normal (I don't use them, so my experience would be unchanged). Seems to be helping a bit. Computer players seem to at least replace their losses now.