You are describing some of the exercises from the Justin Guitar Strumming SOS course. I bought that course and worked my way through it and enjoyed doing it, but it didn't have a big impact.
To answer your metronome question - I can do it for a few minutes but soon something like semantic satiation sets in and I get out of sync. I start to focus on how my arm feels strumming and I can feel the pick vibrating in my fingers and feel the air I'm moving with my arm and I notice the vibration in the guitar body and as my attention moves around, I lose track of the click.
> You are describing some of the exercises from the Justin Guitar Strumming SOS course.
Not surprised that these are in there!
> I can do it for a few minutes but soon something like semantic satiation sets in and I get out of sync
Huh, I never actually did that for more than a few minutes at the time (when just using the metronome), I can imagine it gets boring and attention wanders. I did try to do it frequently (couple of times a day, spread throughout the day), but only for two minutes or so.
Anyway, I thought you might have tried all of that already, considering you took lessons and bought courses, but still wanted to throw it out there. Good luck!