That isn’t how impedance matching works. Is it true that some power is lost in the ATU, but that is only because the inductors are not perfect inductors and the capacitors are not perfect capacitors. There are some resistive and dielectric losses. But an ATU made of inductors and capacitors does not magically absorb reflected energy and somehow turn it into heat. An impedance matching network made of pure inductors and capacitors is lossless.
Well done and thanks for sharing. I would second M1EYP’s advice about using 25m rule. I usually set up just off summit for two reasons: stay out of other people’s way and to shelter from prevailing wind. Good idea to have an easy/quick option like the Hamstick. At the end of the day, if you get the contacts logged, it’s a good antenna. I did expirement with a quick deploy 20m coil loaded vertical last year for similar reasons (still need more field time to properly assess) and current project is a super-lite quick deploy mag-loop.
Had a play today with a similar setup to Tom (M1EYP). It was a sunny still evening. Probably need to extend the guys to bring the radials a little higher at the ends, but the SWR was pretty good between 1:1 and 1:1.5 (without using the tuner). I took a VNA with me but the battery was flat! Got QSOs in the USA and Russia, so pretty happy. Need to test in better band conditions.