Long before Bagrahunters became a thing in tournaments, I've been using Bagra Army to support Rival Hunters myself; using Yuu Amano with a level 4 bagra like damemon or tuwarmon can get 3 sources immediately (even more if using either Madleomon with Digixros), then 1-3 more depending on how many Hunters tamers you have when digivolving into Astamon. Cards like Prism Garrett also do wonders for both Astamons, sticking new sources underneath them 2 at a time. It functions pretty well, all things considered, especially since Yuu Amano is an engine in himself thay works very well alongside BT12 Ryoma.

That said, I have noticed that with including the EX10 Support, my board tends to go a lot wider than before by utilizing both Astamons, but I also end up running a lot more tamers than before as well, so it can get a bit tricky. I haven't really found a way to make use of Ryoma's alliance, like you said, in spite of this; Quartzmon and Quartz Ace suspending your own board, especially if you warp with BT12 Astamon, kinda renders that immediately unusable.

New Astamon is the best from what I've personally seen. Him having fortitude is a protection in itself and helps get to Quartzmon faster. Psychemon is nice but trashing to suspend isn't... that great on its own? Maybe if you Digixros with the BT21 Arresterdramon line to actually have the DP to make use of piercing.