NextCloud is probably the best option but prepare to pay for storage. You’ll need a NAS with at least two HDDs and personally I would also pay for cold storage to have offsite backups in case something happens to both disks.

I’m going down that route now and at a glance, a 18TB setup will cost me at least $700 (local currency) all in all. ~$400 for the disks and another $300 for the server.

Server costs are nothing compared to what a lot of Lemmy instance admins are covering right now. The new price feels like robbery, as someone who got sync the first year it came out.