You are spending a crap ton of money for 3TB (I had to re-read your $589 a month quote!). However, that has benefits, because I understand your budget.

The easiest way would be what other posters suggested, is to expand OneDrive/Sharepoint storage (it is essentially the same thing, but Sharepoint is biz focused, and it changes your licensing as far as I know.) Then, you're not managing anything, which is what software as a service is supposed to be. If you take that route, I would *still* buy a NAS to locally backup data, especially archival stuff from old projects.

Just to verify that I know what I'm talking about due to my low post count, I have 14TB of storage in my house for MP3s, document scans, etc. It cost me about $1200 to upgrade from my old setup, and is fully redundant for drive failures.

Broad strokes:

1) Buy two Synology NAS devices (I have a 1520+, which has been replaced by the 1522+ but is perfectly servicable LINK + )

2) Put three 4TB drives in each (or whatever capacity is most cost efficient, sometimes 4TB costs more than 6TB drives, and ALWAYS buy Networked Attached Storage -NAS- drives, they are less sensitive to vibration and heat, which is a thing when you pack drives together.)

3) Plug both units in at your office. One is primary, one is secondary. Copy all your cloud data to Primary. Once that is complete, configure Hyperbackup to replicate from Primary to secondary LINK .

4) Once initial replication is complete, take the secondary unit home and connect it to the Internet. You now have two hot backups for all of your data (configure HyperBackup to replicate the primary to the secondary, don't forget). I would advise you put both of them on a battery backup ($70) to permit them to write cached data before a power failure.

5) If you're super paranoid, you can also set up C2 for Business by BackBlaze within the Synology interface, which gives you yet another backup. I backup a small portion of my data to C2, and it costs me less than a dollar a month (you read that right) for 200GB or so, for triplicate backup. I don't need it, but it is cheap, and far less than my wife's Door Dash fees each month.

6) Depending on what your "server" does, the Synology line can probably do nearly all of it. It offers (geek warning) DNS, LDAP, SMTP server, logging, etc) downloadable as "packages." Synology is essentially the iOS of operating systems for storage/routing devices. It also has a VPN server as a package.

7)Caveat: I have not used any collaboration tools from Synology, but I suspect that if you're talking about CAD drawings and similar, none of that stuff seems to permit versioning, etc., anyway. Word docs and spreadsheets are relatively easy, any other document format not so much, but your folks are most likely editing MS formats, whereas the rest of the data is reference/archival.