This is what I do. I sync to my NAS, my NAS syncs to my Backblaze B2 with file versioning. My current monthly bill is only $4... I cannot believe how cheap Backblaze has been.
I can even get the B2 URL and share a file anytime I want from inside my NAS.
Is it as easy as Dropbox? No, but I haven't missed using dropbox. I just mount my NAS to my RDP sessions when I need files "synced" between my various computers.
I've been eyeballing this kind of setup lately, because Dropbox has become so hostile to simple use. Hours of delay between "changed small text file" and "synced", endless "bugs" that push things to cloud-first storage rather than the synced folders I have set up, and using more and more resources with every upgrade. I'm so sick of their business-oriented shift.
</rant>
Got a favorite NAS that works well? Without the risk of the vendor's cloud deciding to delete your data? I haven't yet picked one, beyond "what if I just used a Pi and some USB drives..."
I use Synology. Its not the best, but it Just Works and got the "Shell" for only $120, plopped in 2 8TB spinners, and haven't really touched it other than moving house 3 times.
If I was going to do it over again, I would get one of their commercial grade NASes/ They are a bit more expensive but have a decent CPU and RAM in them and can run VMs/containers/etc and be your own little cloud.
They also have some cool features like, the ability to link 2 remote Synology NASes together and sync back and forth as well as backing up to any backup service you want.
Not the person you are asking, but I got some leftover computer parts that I had been amassing for years, bought a few drives and added TrueNAS to it.
It can do anything the big consumer NAS can, and a lot more. If it fails, it uses off the shelf components.
The only thing that could have been done better is ECC memory (for some people that's a dealbreaker). In which case, one can get a server from ebay for a pittance which will fit the bill.
I can even get the B2 URL and share a file anytime I want from inside my NAS.
Is it as easy as Dropbox? No, but I haven't missed using dropbox. I just mount my NAS to my RDP sessions when I need files "synced" between my various computers.