Hi!
I can guarantee you, if the receiving device has SetNextAVTransportURI activated, it will be gapless, not just reduce the gap between tracks.
The thing with UPNP is, both the software and the device need to have that protocol activated... Open Home (a variation of UPNP developed by Linn) is different, only the receiving device needs to be gapless, because the playlist is managed there, not on the sending software.
For Windows, the only device I know it's gapless is Emby - you install the Server and then use either the Windows app, or the Android app or Symfonium to manage it (Symfonium is a great remote for Android, works with a lot of servers). It's been a long time, but maybe Plex and Jelly fin are gapless too, and I believe former Logitech Media Server, now Lyrion Music Server is too (but now I see in your post it's not). One other option is MinimServer and Bubble UPNP for Android, works great too.
Without installing a server, the only other solutions I know are Aurdirvana and jRiver, which have free trials I believe.
Foobar's only properly working UPNP plugin is the continuous stream one (which works great). As you found out, the other one (which is quite old and not developed for a while) is not working glitch-free. Media Monkey is also not gapless.
As for the receiver, most these days have the protocol activated. Eversolo has, gapless works, but it behaves a little weirdly (it loops the last ack). But the Eversolo is here for test, my day to day streamer is a Cambridge Audio, and that is perfect gapless.
Using either Symfonium + Emby or MinimServer + Bubble UPNP I can guarantee you these ones are gapless:
Eversolo (with a few issues not related to actually being gapless)
Cambridge Audio
Wiim
Heos based streamers (Marantz, Denon)
Lumin based streamers (Lumin, Audiolab 9000N, Luxman)
JBL
Matrix Audio
iFi Audio Neo Stream
Volumio
I have not tried it, but know Moode Audio for Linux is gapless too.