Please correct me if i’m wrong, but as i understand it (using for example Vavra as an example, and the explanation given here: MiMi-d - creating a new software synth for Zynthian - #21 by riban)
all that is required is to issue a sudo dpkg -i debfile (which will in that case end up in the lv2 folder from what i can see), then do the “search for …” in the software->engines and/or potentially a reboot and it should be visible?
I tried it but Vavra is not shown in the listing of plugins 
I’m running “Vangelis” on the rpi5 and have Osiris working.