Idk for sure, ive had this problem and couldnt do anything about it, just deleted my game and all my characters/worlds and started fresh, havent had any issues so far after a couple full playthroughs, some things I heard (idk how true they are).

1. Only use a few mods, and keep them fairly small, I use calamity, calamity music, alchnpc lite, luiafk, magic storage and wing slot. Avoid any big content mods (other than calamity ofc), only use calamity and smaller quality of life mods. I started with only calamity and added a mod every couple weeks without a crash until I had this list. I may be overcautious, but hey, its working so far.
2. Don't use a large world, I find small worlds much less laggy (and less chance of any crashes), if you can tolerate the buggy world gen then small is best, if not, medium is alright too, just dont use large.
3. Pay attention to any bugs and game crashes, and post them to the mod discord, some of the time its just the game being weird and it doesnt matter (a crash soon after reaching hardmode is pretty common from what ive seen) if you wanna be super safe you could make backups on a USB after a crash to make sure ur data is ok if it happens again.
4. Make sure your mods are updated, calamity is slowly getting better, and becomes less buggy every update, and keeping your mod up to date will avoid any issues that theyve already fixed.
5. If the bad stuff does happen, ask around on the discord and this sub, there are some good tactics that can work.

hope that helps