I had a regular coaching session on Monday, worked a lot on the BH loop, but something still felt off. Another coach watched me and had me come help him train a newbie. After that he said he notices some issues with my form and thinks he can help me fix them. I then scheduled a 2 hour session with him and wow I learned a lot!
I actually had a lesson with this coach a couple years ago, and it was immensely helpful and actually convinced me that good coaching is actually useful. His schedule didn't fit me then, so I ended up finding a different coach and I didn't want to get potentially conflicting advice from different people so I stuck to him. My current coach is very good at spotting issues with my form, but he's not as good at teaching me how to correct them, so I often have to think long and hard about his directions and come up with ways to fix my issues.
This coach is much more active. Seeing poor form kind of burns a hole in his heart

He's eager to correct it and he has ways to do so. I now need to fix a number of issues on both FH and BH, a bit more so on the BH side. I've made some adjustments on my BH side, all resulted in improvement but now I'm overdoing a lot of it.
For BH:
1) In the process of trying to stabilize my elbow, my elbow actually goes inward now at the point of contact. I need to relax it a bit and not try to stop my elbow motion so hard.
2) In the process of trying to let the ball come to me a bit more, my activation sequence is now a bit off. I'm hitting the ball closer to my body, but I'm not backswinging more, the result is that my racket moves forward very little before making contact with the ball. We worked on both backswinging more (he said to pretend I'm cutting my belly with my racket), and to swing the racket forward slowly to meet the ball before accelerating in a smooth motion.
3) In the process of trying to hit more into the ball, I'm now starting with too open of a racket. I actually had to go through the same process making the same mistakes along the way on the FH side. I've mostly fixed it now on the FH side, but basically I started off by learning how to hit the ball, then closing the racket angle down to "hit" the ball with a closer angle to generate more spin. Before achieving that I went through a phase where I start with an open racket and then close down before contact, which is the stage I'm at with my BH right now. Instead I need to start with a closer racket angle and not change it.
4) This is something I've been working on, and I'm making some progress with it but not fully there yet, which is to relax my shoulder.
On FH:
1) Our main focus was on my wrist. I think my overall body usage is pretty good now, at least when I'm aware of it. He discovered an interesting issue with my wrist, which is its lack of flexibility. It's a bit of a weird issue, but when I hold my wrist at the optimal position during backswing, I struggle to fix my index finger onto the racket. For some reason, I need to lift it off the racket, so I can't really apply index finger pressure during my FH shot.
2) We also worked a bit on my arm motion. I'm not using my elbow flexion enough, and instead use more of my pecs transmit power. The result is that my elbow finishes a bit too close to my body, my racket doesn't cross the midline, and it also finishes too close to my face.
3) Related to #2, my overall motion is still a bit too down to up than back to forward. This is something my other coach pointed out often before, but not persistently. To train #1 and #2 together, he first tried to make me counter hit but keeping the racket above the table. I had a lot of trouble doing that as my muscle memory took my racket down every time. Then he had me to the typical counter hit warm up but over the table, with the stipulation that I can only use my arm and wrist to generate power. Basically I'm doing counter hits using FH flicks against topspin. I can't flick a topspin to save my life, and apparently that's because of the above deficits -- too much down to up motion and terrible wrist usage.
That's a lot of things to fix! Thankfully, he wants me to focus on just one part of the body at a time, and as you can see, all the advice is on arm motion. He doesn't want me to think about body usage and footwork right now, just get these things right. He also wants me to do a lot of shadow training, he says that's the best way to get used to new forms, which I wholly agree. He basically wants me to go back to the basics, wants me to also practice touch by bouncing the ball against the wall, and try to vary and control the bounce at will.