Good point about watering. That’ll help the rootstock you tbud.
Been a few years since I tbudded. I did a large number for several years in zones 5 and 6. I did it at the end of July, early Aug. Late august and it might not heal and harden off before frost. I’d still try it if it was the only option. I always did 2 buds on each rootstock.
If the videos didn’t cover it, clip the leaves off the wood you collect to leave 1/2” to 1” of the stem. That’ll prevent loss of moisture from the leaves. Wrap in moist paper towels in a ziplock bag, keep cool and shaded/out of the sun. Should be good for several days. I’d collect in the early morning if you can rather than the heat of the day.
There are probably some old posts showing what I got from the USDA GRIN. They shipped UPS overnight when they were shipping for budding. They haven’t done summer wood for a few years because of fireblight problems.