As I said, the desync for MP are a known issue with (as far as I am aware) 0 current fix.
Paradox likely knows about it by this point (given the amount of posts here in the forums on the subject)
We haven't seen or heard anything on a fix for the problem
Here is what I can suggest to try and stem the problem;
1) Start a fresh game
2) Use a Medium Galaxy
3) Use less empires [i.e. try and mitigate how many ai/fallen empires exist in the game based off how many people are in your mp session, recommend not going above 4 ai empires and 3 fallen empires to test things out if you have 3 other people in your lobby]
4) Use other default galaxy settings [1x the hyperlanes, gateways, wormholes, habitable worlds, etc]
I can't guarantee if that will help with the desyncs, but honestly trying something is better than doing nothing