I'm not very experienced at all, the game for this jam being the first real shmup I have made, so as far as tips go all I can say is what I've done for this jam: play a lot of shmups, write down their patterns, analyze them, try to recreate them, then put together a combination of what you like in what you think is an appropriate difficulty curve. Introducing individual patterns one by one then mixing them together once they've been introduced probably helps making a good curve. I got help from more shmup-savvy friends, even if they aren't devs their knowledge goes deep.
Best of luck.