I have also experienced this, playing on easiest difficulty as rebellion and getting a 400 stack of tartan patrol cruisers, marauders, and skiprays shoved down my throat, no lube. Started a new game as empire and didn't have as much of a problem with the other factions doing this.

I tried playing the rebellion on the hardest and they essentially double the doom stacks of skiprays and the missile corvettes

This is certainly an AI issue. I'm no coder but I do understand the AI and why it's doing that.
In a nutshell the developers recognize that the Empire is a popular choice so that faction is given a harder start economically. On the other hand, the Empire has access to some of the most lucrative single-planets in the game and just enough ships to defend it.

In order for the AI to be a legitimate threat with such low starting income, they with spam the living sh*t out of corvettes and fighters until a new tech lvl is reached OR expanded their control to field larger ships. Fighters and corvettes do NOT suffer upkeep, only population, hence the doom stacks despite controlling a handful of planets.

My Recommendation: Wipe them out...Fast. Reduce the faction to one planet and corner them. This not only reduces their income but the population cap as well, completely negating the potential for doom stacks. It won't be easy, but the sooner you do this at the start of a new game the easier the mid-late game will be.

Check out my space battle guide more in-depth tactics and strategies.

Cheers!

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2780215273