There’s a lot more to the genre, and the isometric shooters you’re talking about are technically not shmups, rather they’re twin stick shooters. A true Shmup is the bullet hells you mentioned, but bler hell is actually a subgenre, there are many types of shmups that control the same way a bullet hell does, but minus the hell of bullets. Check out Shmup Junkie on YouTube and join his discord to learn more than you ever imagined about this amazing genre!
Synthetik isn’t actually a Shmup, if you’re looking for games like it you want to look into twinstick shooters. A Shmup is an automatically scrolling shooter with where you are traveling in a single primary direction. A true Shmup gives you freedom to move all around the screen but the screen scrolls through the level at a fixed rate. Famous shmups include Gradius, R-Type, Raiden. There are a few different subgenres but the main offshoot you’ll see is Bullet Hell.
A great place to start is the Touhou games, and fan games (My favourite is Blue Devil in the Belveder). My all time favourite Shmup is Teslapunk, but it’s pretty obscure and not exactly popular.
If you want to learn more look up Shmup Junkie on YouTube and join his discord. Lots of great guys and a curated list of beginner games/goals for someone who’s interested in the genre. Trust me there is a Shmup for every taste out there. Try lots to see what clicks for you!
Number of hours doesn’t really come into it these days compared with how fun the game is for me. I’m nearly 40, and whether or not a game is engaging is most important. I’ve got about 50hrs into Avernum: Escape from the Pit (a retro style isometric RPG), but I’ve got nearly 80hrs into Teslapunk, my favourite Shmup(completing the game takes about an hour).I Iove Dark Souls 1 and 3, Bloodborne, and Sekiro, but eventually got bored of Elden Ring and its open world.
Ultimately how fun a game is combined with how painless it is to get started is what I’m most interested in these days. I don’t have enough free time to be worrying if there are enough hours of gameplay.
I beat the original dark souls without realizing there were different weight thresholds for rolling. I fat rolled the entire game. Also didn’t realize boosting vigor was important for hp. I did 99% strength/stamina and only as much dex as required to weild my weapons.