Outdoor training with a different power meter

I do all my TR workouts on a roadbike with Assioma’s, most of it indoors with a Kickr using PowerMatch, but also the occasional outdoor workout. This works very well for me but unfortunately I need to replace my BB now and the LBS is waiting on stock, so no roadbike.

I do also have a MTB with a Stages Gen1 powermeter on it which, based on my testing (actually put the Assiomas on there to compare and recorded with two headunits, had some spare time during lockdowns:-)), reads on average 20W higher.

My question is, what is the best way now to perform outdoor workouts now (I cannot put the MTB on the trainer). Is it enough to up my FTP on my Garmin headunit with 20W and just send the TR workout there? Or do I also need to increase my FTP on the TR side? If I need to change my FTP on the TR side, won’t that mess up my progression levels?

Another option I guess is to put the Assiomas on my MTB, but I also do some trail riding and constantly swapping pedals would get old very quickly.

I do most of my stuff on the Turbore with power match from by Favero Be Pro S pedal on my TT bike and usually ride outdoors with that but similarly I’ve got a 4iiii crank on my road bike and if its less than perfect weather I tend to do outside workouts on it. There might be a discrepancy but I’m still training so I don’t bother about it, life too short. One day I might see how the 4iiii compares to the Favero Be Pro S and offset the 4iiii to match (I don’t know if Stages let you do that). But like I was saying life’s too short (I’m not training for the TdF) and there’ll be other FTP discrepancies due to the road/TT bike thing anyway.