I've just finished setting up exactpy what you describe (Synology NAS and Debian mini PC server) and it is working so well. Why do you need a more powerful NAS? If you use it for just storage you won't really be falling short of NAS compute power.

I considered adding the drives directly to the mini PC as well, but I'm happy I've kept the Synology for the best of both worlds. Synology makes it so much easier to share the storage, set up reverse proxy, provide constant 2 way sync with my online cloud WebDAV service, etc.