A non-Apple NAS will never be reliable, and Apple discontinued the only NAS that was: your TC. They're worth keeping in service if for no other reason than that. Its internal hard disk drive can be replaced with literally any standard 3.5 inch desktop HDD. Disassembly isn't particularly difficult if you are so inclined. But whatever you do do not get a non-Apple NAS. It won't work. Consider yourself warned.
NOS Time Capsules are readily available on a popular auction site, at least they were last time I checked. Prices are still reasonable but climbing as the supply of them diminish.
As for other options, literally any external USB / Thunderbolt / etc hard disk drive will work. I do not know of any that won't. No manufacturer is more or less reliable than any other. Speed is not particularly important. Personally I use drives pulled from discarded Windows PCs because they are abundant and literally free. Install them in an inexpensive (less than $10), slow, USB enclosure. That way you can have as many additional redundant disposable backups as you want, keeping at least one of them geographically distant from the others at all times.
You can also connect as many of those cheap HDDs to a Time Capsule as long as you use a powered hub. And the TC has to work (obviously).
Another thought is to buy a NOS or used Apple AirPort Extreme Base Station. It has to be the last model produced by Apple (the 802.11ac "tall" model) for it to be officially supported. Connect those cheap HDDs to its USB port the same way—through a powered hub. The AEBS does not even have to function as a router. It can simply be a client for the sole purpose of hosting those TM backups. Same for the TC.
