My Wi-Fi is awful.  I have the AX21 upstairs where my internet coax enters the house.  That did not give me good coverage so I added three RE505X extenders.  I get strong signal now everywhere I need it, but...

The interference is terrible and it's unstable.  Devices get on the wrong extender.  Devices drop off.  It's a mess.  I used the Tether app and it says the extenders are in "perfect" locations.  Wi-fI Analyzer says the interference is 90% and I should switch channels.  There is NOTHING else on the channel.  I switched channels from 1 to 6 and of course the interference just moved over as well because it is the whole "mesh" system interfering with itself. Changing the power level didn't help.  This "mesh" should be smarter than this.

Any ideas on how to make it work right?  I'm ready to throw it in the trash.