I suck at shootemups. What do?

Git gud, I know. Any other tips?

Did your computer get hit with the Touhou ransomware and you aren't good enough to beat it on lunatic to remove it?

I would say figure out what style of shmup is best for you. Some people just stay away from bullet hell shooters and that's ok.

I would start with some classic console shmups that are just based on enemy placement memorization and patterns. The console ones tend to be better to go through than arcade ones because they're designed to be finished within the constraint of a certain amount of lives and continues, but on the arcade you may be tempted to just activate another credit and continue. A console shooter might be more fair also instead of trying to suck quarters.

Since reflexes are important I'd make sure you were playing with the lowest latency possible. Some retro game setups allow for a "runahead" mode which can combat lag even on displays with some latency and this can be ideal.

Some very popular early shmups like R-Type and Gradius have great guides already and that might be a good starting point. People already know about strategies, and you can note "safe spots" on the screen where you can escape to if things get hectic especially during boss fights.

Additionally, you can play with some amount of slowdown at first to master difficult areas, and then slowly speed things up to normal.

If you want something more modern that can be easier to play, look into Jamestown, which has an upgrade and progression system. It can make you feel like you are chipping away at the game and getting further, and it looks and sounds fantastic similar to a Neo Geo game.

Record some footage and see what you do wrong is usually my answer. Self reflection is the best teacher for improvement.

Cheat and savescum, don't tell anyone.

I appreciate the little extra mile you put into your recommendations!

@DEVILinIRON: No problem! Have fun!

Coffee, patience and one eye open

Just play on easiest difficulty.