I've been using pcloud. They do one time upfront payments for 'lifetime' cloud storage. Catch a sale and it's ~$160/TB. For something long term like backups it seems unbeatable. To the point I sort of don't expect them to actually last forever, but if they last 2-3 years it's a decent deal still.
Use rclone to upload my files, honestly not ideal though since it's meant for file synchronisation not backups. Also they are dog slow. Downloading my 4TBs takes ~10 days.
Would you consider making the LLM/GPU monster server as a gaming desktop? Depends on how you plan to use it, you could have a beast gaming PC than can do LLM/stable diffusion stuff when not gaming. You can install loads of AI stuff on windows, arguably easier.