Selfhosted

If your Synology NAS is in Raid, up-sizing your drives is dead simple.

Pop out one of your current, smaller drives. Pop in a new, bigger drive. Wait for Disk Station to migrate data over to the new drive (note that this can literally take days depending how much data there is). Then do the same with the next old, small drive and the next one until all old, small drives are replaced with new, big ones.

This will work as long as your current drives are not like 24 TB or whatever the upper limit is these days. In that case, you need more bays.

After this is done, move all the data from your externals onto your Synology NAS.

Upsize your NAS drives. Get rid of external drives. Set up one nas to back up to the other, or pay for cloud backups. Synology cloud is relatively cheap.

Replace NAS drives with larger ones and then repopulate data from your backup? That way you get to test your recovery and restore procedure at the same time.

Depends a lot on budget, space, and electricity costs. Going to 'overkill' level once can save a lot of these issues down the line.

Mine started similarly, some small box with a couple drives that got up sized and then moved to another to add more...

Eventually I bought a used 2U box with 14 bays and set it up with a ZFS pool all made up of mirrored disk pairs and auto snapshots so it can have a drive fail without issue and go back 2 weeks if something gets oops deleted.

Downside, now the whole lab uses about 700 watts continually so the power bill is kinda nuts.

I have 2 servers. One is used as a NAS and hosted stuff. The other is just for backups. If the space runs out somewhere, it will simply be expanded.