Having abandoned Roon quite some time ago, mainly due to very heavy storage space of duplicate metadata just got to me.
Have you tried Minimserver at all? There's a free version and installs on Linux, even on some NAS directly.
Minimserver is true DLNA/uPNP very lightweight. For control point use an android or ios phone, flavour of the month is JPLAY for iOS that controls Minmserver traffic to the endpoint renderer in your case PI devices. My track count is similar to yours, about 10,200 albums, and Minimserver can handle this load quite well. It's on a Windows 10 system, but no reason it can't behave the same for Linux.