I finally had some time to spare and try your solutions. Apparently the very first worked fine and I was able to play around 2-3 hours, closing and restarting the game on purpose multiple times.
The saves seem to work well now, and the performance also improved, so I guess it is solved! A huge thank you to you for helping me and having the patience to wait my answer.
P.S. Funny thing: I was in my third patrol of the campaign, and I lost multiple times the latest save. So I got back to the first save game working and I had something incredible and never happened before: about 10 merchant ships sunk and a random HMS Nelson while she was sailing to Scapa Flow. I think the game was asking for forgiveness
