I hear you about trimming being hard on your back! Iāve been guilty of getting on one knee or using a stool for the ones I know are generally good eggs. Not the safest choice, which I own, but it saves my back.
A hoof jack/stand would be a very good toolāpersonally, my ankles are shot so I canāt hold feet between my legs. I need them to cooperate with a hoof jack to some extent, and I can use the hand not holding a rasp to gently brace the foot. Youāll find the easiest way that works with your conformation in time.
What brand of rasp do you have available to you? I started out with a Heller Red Tang, but have switched to Save Edge (they might be out of business now, Iām not sure) and now ultimately a Bellota Top Sharp. I do also have an angle grinder that is a lifesaver on some of the big lugs I trim! I also really like my little bonsai nippers for trimming frogs, bars, and heels. Iād use them here to clean up the bars a bit and lightly trim some of the heel back. Iāll see if I can doodle on the pictures to show where Iād use them.
Good/bad news, in the photos the wall separation doesnāt look as bad as I expected If you can get your hands on a hoofjack, or can comfortably draw the foot forward against your thigh, I would probably try trimming some of this from the top. Especially while he transitions, you donāt necessarily want to take height, but you do want to bevel the walls. Iāve found sometimes this is easier to accomplish from the top because you can put a bit more force into it.
Do you have something to help tackle the thrushy looking frog in the left picture?