No real answer to your question, other than what’s been suggested above, but I feel your pain.
Of course you can try and add metadata to your mixes and try to enforce some level of tagging (no easy task, especially if you want to Respect des fonds to use archive theory parlance), but at the end of the day Roon really isn’t build to handle this kind of content well.
Which is a shame, as obscure singles and DJ mixes tend to be exactly the the kinds content that isn’t available from Qobuz/Tidal and hence why you might want to maintain those files locally and therefore want a product like Roon to manage such a library well.
I opened this ticket a few years back, but it’s had vey little traction. My guess is most Roon users, and possible the Roon product team themselves skew heavily, if not solely, towards album centric listening.