I have looked briefly at Custom Tag Importer but the documentation isn't great, and I don't think the customisations are tightly integrated into the standard browse tree, but I'm sure that can cover off some of the customisations.LMS is very flexible so far as local media is concerned if you install some specific plugins that expand the basic capabilities but for most people the standards are fine. I think your extensive tagging and work to modify Minimserver to your requirements could be replicated in LMS but it would take a lot of work. That’s what I mean by your use case.
So attractive, that in the 23 years since its inception we now have one manufacturer that has chosen to support SqueezeliteYour point about UPnP being in use in most components is of course true but it is the very fact that hardly any two implementations are the same that makes other totally predicable protocols a viable and attractive alternative.

On a serious note though, a modern open standard is well overdue, but unlikely.
The WiiM's are first and foremost streaming endpoints, so to get the best user experience you're always going to have to use third-party apps, but I don't think that's any different from any other system really, and I think it's only going to get worse with companies keen to own the interface (advertising space)/data.My view remains that as matters stand UPnP remains virtually unusable on a WiiM with only the WHA as a control point. You need BubbleUPnP or mConnect to do anything more than basic play. Given the vagaries of the UPnP implementations by various device manufacturers I can’t see this situation ever improving.
I do appreciate LMS but its sprawling nature makes it feel/look very fragile.