I used some nearly flush mount rivnuts in some places. Due to the reduced surface contact, I thought some loctite bearing and sleeve retainer would assure the rivnut did not decide to spin at a later date. Only time will tell, whether this was needed or works. I also found there was enough metal in the thicker wall tubes to use a self tapping screw/bolt.
Just throwing out some of my thoughts. Not that you need any, looks like you plan very well. Nice work.