I'm honestly considering this myself. I've started buying copies of the movies and shows I really like when I see them.
I do know most physical media decays though. Not sure what the best way is to cope with that, but I've never been a huge fan of streaming in the first place
Once you have the physical media, ripping it to a file and using standard backup practices is really your best bet. It's a bit roundabout, but 4K on disc is significantly higher quality than the compressed bandwidth saving versions available via streaming services.
You can also download remux quality rips. Be warned: they take a lot of disc space! Hard drives cost 1.4¢/GB right now, so say 2¢/GB for a raid NAS, all in. That means each remux costs about $1 USD to store with backup... But you need to put down the cash for like 1000 movies at a time to get the best marginal price.