I personally love having so many fighters, and never have any frame rate issues and all I have is 4 gig vram, 9th gen i3, and 16 gig ram and m2 ssd
Your issue could be somewhere in your hardware: you dont need expensive just the right pieces.
I'm running a Ryzen 2700X, 32gb RAM Dual channel a 1660, and also M.2 NVMe PCIe SSD, so my spec is not that far from yours.
as for the heap size, with all the mods I use for Nex and factions mods, I will not even start up at 6GB heap at times thus the 8GB heap
and CMIIW here but I believe you were recommended a 6GB heap because the JVM Process Size or Memory Footprint will typically be larger than Maximum Heap size (upwards of 1.5x) plus accounting for your OS RAM usage, background processes etc, thus the 6GB heap out of your total 16GB RAM.
Sure when running on normal speed its fine with a few slowdowns when moving cameras etc, but using Speed Up you will get slideshow. Keep in mind this was a fight with a lot of fighters/bombers/interceptor with each capital/cruiser having 6-8 bays versus a hegemony bounty using 5 carrier capitals and a quite a few carrier cruisers
not using speed up is a slow and boring as hell, sure I can probably take it the first several fights with normal speed like that, but running a long campaign, I don't think it'll be fun....