How to keep adding on

So like many of us, is started out small, and have been progressing to needing more storage. Right now, I have 2 synology nas devices and a lot of random external drives (yay adhd).

My question is, when i need more space again, should I buy another nas, or try upgrading my drives to larger ones? The issue is find with upgrading is i wont have a way to move all my files from the smaller drives to the larger drives.

I was also thinking I should have another server or something to back up both my existing nas. Not sure. I worry im not doing it right.