Your concern about disappearing music on your playlist is justified. Consider artists leaving streaming services (yes, it has happened: see story for instance here https://www.midiaresearch.com/blog/some-artists-are-leaving-spotify-again-heres-whats-different-now), a new remix being substituted for your beloved OG version, etc.
I only play purchased digital files locally. If you play minor, living-active, artists, there is also the aspect of supporting those artists with your $$$ as opposed to some corporate exec salary.
Re local storage, there are packaged options like the nook, aurender etc. or you can go home grown. I use a Mac mini running Roon. Then you *may* be able to feed the local files via USB into your existing streamer. Alternatively, you can also only get a DAC. With computer - DAC route, the advantage is separation of components and independent swapping, the disadvantage is having more components. Typical trade-off situation. I want more flexibility with occasional digitizing vinyl, so need a computer anyway.