Yes that's what he does - he serves backspin higher and topspin lower. I also can't rely on the bat angle because he can also serve sidetopspin with the flatter bat angle. From my perspective, if the force is directed downwards, the only way topspin can happen is if the bat angle is more towards 90 deg, otherwise it's always backspin. If the force is upwards, the only way backspin can happen is if the bat angle is very flat, otherwise it's always topspin. The followthrough is always very misleading for sure.
I like what you mention - having a mental photo of the contact. It's something I don't do 😭. I don't have photographic memory unfortunately - how do you train this up?
You don't need a photographic memory. As in all things in life:
1. Breathe so that you are not tense, and your body isn't in fight or flight response.
2. Focus your attention broadly at on the server, especially looking for when and how the ball is contacted.
3. Whether you see the contact or not, your body will pick up cues. If you play a best of 5, by game 3, whether you like it or not, you will be guessing the spin better as you watch the contact and the ball better and better.
The main issue is the quality of return and how you play on your serve, You need to have a quality return to use on the serve with a forehand flick or deep push or even a short push, so that when you get the spin read right, you can stay in the point. What happens to most people when they cannot read a serve is that they fail to play a quality return either early or late (a late return is not high quality, but it should still be quality, mostly likely deep even if high so you have time to step back and defend) and then this leads to them pissing points even when they read the serve right as low quality returns will get you into trouble against a good attacker.
The other thing is letting the anxiety about the score hurt you on your serve points. Don't feel the pressure to do anything more than to probe the opponent's game for weaknesses. If you can keep almost level on your serve points, you just have to get a few of his serves right to win the match. But you don't have to do this immediately in a best of 5. And you don't have to feel the pressure to do anything special on your serve other than just execute your game. If you lose to him, consistently, he is likely better than you at the moment, but it will even out over time.
Look for serving close to the tip versus closer to the handle. closer to the tip tends to be backspin, closer to the handle tends to be topspin. Also, even with an open bat angle, it is possible to impart topspin on the follow through by coming up slightly on the other side of the ball. This is where looking at the contact closely helps, because even if you don't see the precise contact, combining the contact with the quality of the ball trajectory usually gives you the information you need. because even when he fakes the follow through, you will feel it in the timing of the contact.
If you haven't already tried it use the TTEdge app for a bit. It will tune you in to photographing the contact mentally. The body adapts to anything with practice, you just need to practice. But just as important is having a quality return against the basic serve spin.