That sucks, similar things have happened to me, it's part of the game after about 100 years there will be a 'mid-game crisis' and after 200+ an 'end-game crisis' which may well smash you down. They've added a lot of sliders and options in the difficulty settings so you can play on easy with aggressive AI or hard with passive AI etc. Doomstacks are partially gone as there is now a per-fleet command limit.

I have all the major DLCs and most of them are worth the money, at least compared to any other Paradox DLC.

My main complaint is that unless you can really involve yourself in your star empire and/or roleplay everything gets stale, it's like playing every game of EUIV as England. Sure you can change governments or use different strategies but it's still the same base.