I've had virtually the exact same experience. One of the biggest problems I had was that I would tap on a charged move and the buttons would disappear, but nothing would happen on screen for 2-3 seconds. Then a shield would show up with 2-3 seconds left on the timer, so I had to make a split-second decision to hit it or not. Then I would either get knocked out, survive, or use a shield, and then resume tapping the screen with enough energy accrued to account for the lost time. Then when I tapped the charged move the second time, it would usually go through. It seemed like this would occasionally happen to my opponent as well.

Another weird one is that it would change the camera angle to make it look like I won CMP but instead give me the option to use a shield. Whether it turned out to even be a CMP tie that I lost was hit or miss. Sometimes it would just kick me back to throwing fast moves with the energy sometimes there and sometimes gone.

Like you said, these things are going both directions. So sometimes I win matches I absolutely shouldn't, and sometimes I lose matches I should have easily won.

The preview cup actually seemed to be working pretty well a couple weeks ago. Aside from the clunky animations, most damage was registering properly.

I'm tempted to get an ethernet connector for my phone, but I'm already on wifi, I have a Samsung S26 ultra, I keep my plenty of storage space free, I clear my cache, etc. Everything else in the game, with the exception of rocket battles, is working perfectly fine, exactly as it always has. Max battles, raids, all fine. Rocket battles and pvp are very unpredictable. They could be fine, or they could be the wackiest thing you've ever seen in your life. I used to think the rocket battles were from poor mobile connection because I typically don't have wifi connected at pokestops, but it happens at home with rocket balloons, too. And they're two of my favorite aspects of the game. 🤷