A lot of people have suggested charging clients for long-term storage. I agree with that sentiment. If you go this route, you may be able to use cloud storage a la Dropbox/gDrive - which seems most convenient for you. Costs for consumer-facing cloud storage run roughly $10USD for 2TB. Expensive for hundreds of terabytes indefinitely, but if a single client needs access to (idk) 0.5 tb you could easily charge $30-50 a year to provide them a shared folder in google drive. Maybe more if you want redundancy against the cloud provider losing data.

For anything you need to actively use for work, a giant NAS is probably your best bet. Those YouTubers you’ve seen also use it as part of their team workflow, and maybe that’d also apply to you anyways. You should probably run a regular backup job of these to the other office or to AWS/backblaze. Should be manageable cost if you only need 10-20TB of data for active work.

For everything else… maybe tape if you really want to keep everything. A lot of big organizations seem to be moving away from tape towards networked spinning disk as the price drops. Seems mostly driven by tape being seen as a massive pain to use (not that I have personal experience with it) and expensive equipment. It’s really an organizational decision to directly quantify long term archival needs and value. Once you have a $/TB value to the business, see what fits your budget (could be nothing!) You could try Backblaze or AWS glacier but those get expensive and the cost is ongoing forever.

There are a whole bunch of niche and small-scale companies doing cloud data storage, but I don’t know how they’d get lower cost per byte stored over some big companies (lower margins? Slower speeds? Lower guarantees?). I’d be suspicious of them for mission-critical storage. It’s one thing for a home-user to use them to store their torrented movies, but it’s very different for a business. It could be worth it to just search around. Look at what’s supported as a target by whatever NAS software you use if that’s your route.