Are there any missables?

Nope. After you beat the game you can load your save and do whatever you missed.

Any codes for bottles?

These are hardcoded, in fact, and you can use one of these tools to get what you want:

Some things are limited to the bottle you get in postgame, including two optional bosses that only appear in worlds labeled as Domain and Ravine. This is also the only way you can acquire the Underworld Master item. If you’re just after mats, you don’t really need to go for the high level stuff; while higher level will mean better potential traits, it’s not important unless you are actively trait farming for ++. Resonance Orbs are more likely to appear multiple times in higher level bottles, and, indeed, do not appear in all bottles.

I don't really understand synthesis.

See this page.

[PC] Why does my game run like garbage?

Plug in a controller. Yes, really.

Alternatively, if you are on laptop, you need to make sure the game is not running on integrated graphics.

Windows 10 Settings -> Search "Graphics settings" -> Browse -> Select the game exe -> Options -> High Performance -> Save