How to defeat Fallen empires (4.4, no DLC)

I have seen plenty of old videos on youtube where it seems almost easy to do, but those strategies just don't apply anymore in the current version. I also saw a good guide for 4.3, but that one heavily relies on bioships + tech rush with synaptic lathe which are both DLC content.

So my question is: Is it even possible to beat fallen empires in the current version, vanilla, no DLC.

I have tried once as a driven assimilator, small galaxy, captain difficulty, endgame start 2350, absolutely steamrolled all standard empires. I had about 1K alloy incomes towards the end and 1.5K fleet cap, which I maxed out with battleships (carrier + X slot focused arc emitter, which I saw in a recent guide from Ep3o) (although I did not go over cap significantly, due to one battleships being 4K alloys each, and ramping to 1K alloy production took a long time). I built citadels at every chokepoint next to the FE (it was the xenophobe one).

With my fleet (which was about 250K by then) i managed to take 2 systems, then they awakened, spawned a 600K fleet and steamrolled my fleet and citadels. But even if they did not awaken, they still had a nearly 300K fleet + those crazy strong citadels in every system + even if you defeat their fleet, most ships just disengage to fight you again soon.

Maybe I am just bad at the game and don't know how to properly ramp economy to pump out 20K soldiers and 5K alloys or maybe it is extremely hard to do by 2350 - could well be the case - but regardless, it seems very hard to nearly undoable to defeat an FE in 4.4 basegame, so I'd like to know your thoughts and strategies.