Hi all! Thanks everyone for the feedback.
This was particularly helpful:
Ry & Scooter said:
There's a few things you could try.
If you're using a larger needle like 29 gauge, try using a 31 gauge. The bigger the number, the smaller the needle. 31 is the smallest you can get.
Try using short 8mm needles instead of longer half inch ones.
When you pull the skin up to tent, make sure you're injecting PARALLEL to the cat's body, not straight down into the tent.
If he's a good eater, shoot him while he's eating. It will keep him still and distract him from the annoyance of the needle.
Good luck!
I will pay extra attention to being parallel (maybe my technique has gotten sloppy!), and try right when I give him food. As far as needles, the ones I'm using are 31 and pretty short. Had to check that though

Part of the problem might be that he's started grazing more rather than eating the whole can at once, like he did when he first started canned. Back then I would shoot him up about half way through the meal to make sure that he was going to eat and not get hypo, but now I try to do that and he stops eating like a quarter of the way through the can. He eats one can twice a day, btw.
Please allow me to clarify all that has happened since my original post.
Yes, Max is a bit over 16 lbs. He started on 2 units and then I took him into the vet for the glucose curve. The vet decided to up the dose to 3 units, which I was nervous about, but I upped it anyway. A few days later he was vomiting and had diarrhea. I took him back to the vet and they ran hundreds of dollars of blood tests, which I was visibly frustrated about and the vet seemed to realize this, because after about 45 minutes sitting and waiting for the results, this other vet came in. He was awesome, he was very reassuring and he actually suggested I buy a blood monitor so that I could manage him at home with him on the phone! I was so relieved, and I got the monitor. We changed the food to i/d which is for cats with sensitive tummies and I got him on some probiotics and backed off on the insulin and the diarrhea and vomiting stopped. We had to work back up to 2 units per meal. His blood sugar is usually about 200-220 in the morning and around 180-220 in the evening with the 2 units, so I'm not really worried about him crashing, but I always check.
I do feel like Max and I are in much more capable hands now. The other vet totally blew me off when I asked her about home testing. This one suggested it all by himself, and for a few weeks called me every morning to ask about Max's numbers. Obviously he can't do this forever because he's not charging me for it and he has a lot of other patients, but I thought that was really cool.
Anyway, my question about the struggling. He is really good about the ear pricking. He's obviously not a fan but he tolerates it. As for the injections, I was doing it fine, and for weeks he was tolerating it, and I never restrained him, cause he hates that and I didn't have to. I always take the Lantus out of the fridge a bit before and warm it up in my hands and stuff. And I was never nervous about it until she started the twitching, so I don't think that was what caused it. So I dunno. Maybe I should try it in other places. I know those are supposed to be better injection sites anyway but I also hear they hurt more.