The "DamageSharing" issue already was in earlier Sins versions. I recognized it when helping on the Maelstrom mod. In latest Sins versions (can't remember since when) it wasn't an issue, I only remembered it when reading the complains about Advent. So I run some test games and I'm pretty sure it is this ability (again).
On the crashes:
I'm modding around myself a little, nothing serious but enough the savegame wouldn't help(?)
I don't think it is the PC. I'm also running another CPU & RAM heavy sim game, but also I can run medium maps with 4 to 6 players without issues. Where to say I busted fleet support to 3000 and more and have large fleets battling around without a problem other than slow down due to many thing going on. So the crashes seam really very specific to vast & huge random maps, independent on what the (AI) players are doing.
Now just thinking I recognized a new "hard" limit on ships. I wanted to test the limits on the new engine and pushed fleet support to 6000 which caused a crash when multiple AI would reach this. In 1.85 sins would slow down to a crawl but not crash. I wonder if there is a new "limit" on planets or planet related abilities/buffs that causes trouble?
Probably something that just kicks in on new game start. I never was able to repeat the early (few seconds into game) crashes, even with a save games just a moment before the crash. When loading the game again it runs for an hour or so before the next crash. But as said only with very large maps.
Now even with the Dev exe and using record/watch to check any used ability, I was unable to locate the issue. So it must be the PC or an engine bug.