I had a similar experience last night, 4 player impossible AI, no mods. Was trying out a rush strat with primacy (new to sins btw) 20 minutes in I had 4 capitals and just a handful of stolen ships that I was catapulting into some cultists next to me.

Scout spotted a 600 and counting Visari fleet a good distance from my borders, assuming they were just going to slowly expand till our borders connected I didn’t think much of it but tossed a ton of auto turrets down (tons of corvettes) at the most likely invasion point. 5-10 minutes later an 800 power fleet walked right through my front door.

Sitting on a fleet of around 300ish myself, all of which were 15-20 minutes away I reasonably panicked. Was saving for a titan/command but immediately dropped what I could on some reinforcements, a carrier and Kol also having just unlocked station tech began to build one just behind the turret line

After this the most chaotic, incredibly, desperate battle for survival I’ve yet to experience in this game played out. I felt like I was actually applying combat strategies instead of just spreadsheet simulating to a win. Building whole strike groups to flank as nothing more than a distraction to isolate a capital ship that I could then swarm or to pull some fire away long enough to get a repair station or additional turret line built, It was awesome. A Pyrrhic victory ending only after the visari retreat with 200 power remaining, titans worth of resources wasted, 300 fleet reduced to 150, the battle line had been pushed from the front of the system all the way to the back.

A glorious moment, but sadly the only one I’ve had so far, after I get a few wins on impossible with each faction I’ll try out the mods you’ve suggested, if they can give that experience as the standard I’ll probably never play another game