But I've no shortage of old routers with USB connections, and the router not having an Internet connection is probably an advantage.
Watch out with that. It is very easy to create a "double NAT" problem in your network when using old(er) routers in your network. While you may not notice connection issues immediately, it won't take take long before you'll encounter vague issues, on which you'll spend hours troubleshooting and not to mention visits to the hairdresser for fixing the hair you pulled out during those troubleshooting sessions.
If you really want to have a go at storage, you could try solutions like FreeNas or ProxMox on a spare computer (a desktop with a few SATA ports on its mainboard). You'll only need a monitor keyboard and perhaps a mouse during the installation of such software. Afterwards you can "kick" that computer in the proverbial (ventilated) corner and more or less forget about it. You can then manage your storage using your browser.
Initially much more work to setup and yes, it requires time and consumes electricity, but you'll have a nice and fast storage solution for your whole network, which is easy to expand/reduce to your needs. It is also easy to sync data to/from your all your local devices, to automatize this syncing and making (incremental and/or differential) backups from the collected data on this spare computer onto a portable drive for offsite (or protected on-site) offline storage. A central storage location makes it also much easier to filter what you want to backup into the cloud.
Went through those motions myself practically 15 years ago. Used CentOS (Linux) in the beginning using 1 boot drive and a software RAID (spread over 4 drives) setup, but ended up changing that to Ubuntu Server about 10 years ago as the mainboard of that computer failed. Changed the mainboard and case (for better ventilation internally), mounted the drives again and because of the software RAID it took little time to softwarematically mount that into Ubuntu and ready to run again. Last month I changed one of the drives in the RAID setup for a new one. That and an annual cleaning of dust has been the only hardware maintenance I needed to do. Pretty much set and forget.
FreeNAS, ProxMox and similar software have made all the above much easier to setup, easier to use and are easily as reliable as the setup I created. The initial extra work has given me reliability and with that came a piece of mind that easily justified the costs of the barely increased energy bill.