My 2 cents
I've been using MC since around 2015 and use it every day, especially for:
1. managing a large, local music collection and creating custom views;
2. Playing music around the house through JRemote running on iOS.
I've tried Roon and really like the user interface, but I didn't renew my subscription since it's nowhere near as flexible as MC for managing local content, and I don't use streaming too much (I juste use the Spotify app for that).
However, I must admit that Roon excels with:
1. the simplified use of filters (as opposed to "panes" in MC, which don't transfer too well to JRemote);
2. the dedicated artists, albums and genres views and the clickable links between them.
For my second point above, I think you really are onto something with Spotlight
I dream of a Spolight integration using local metadata (MC fields) OR subscribed metadata, if needed. Using Spotlight through JRemote and navigating between artists, albums and genres would be the icing on the cake