You’re welcome Hendrik.
I think the rule of thumb is around 10-20k rows in the whole doc. If you don’t have many heavy formulas it should still work, but before reaching that point I would consider looking into archiving the old data somewhere else.
Having many attachments shouldn’t be an issue according to a Codan
And if you want to have backups in other platforms you can easily upload the files with OneDrive/Google Drive packs.
Hope this helps,
Pablo