Let's look it from a different perspective. Instead of simply looking at the iPhone being not good enough to do some shots that you want, maybe you can rethink about what you can do with an iPhone to get those shots.
First of all, which iPhone that you have?
For low light shots, night mode on the iPhone 11 series are quite amazing without further edits. And by using a tripod, you can set a longer exposure.
For action shots, take advantage of burst mode.
For others and manual mode, there are 3rd party camera apps that allow you to have manual controls and even save in RAW. Apple's camera API is really amazing for what it allows developers do.
Getting a pro camera app will at least let you be familiar with things like ISO, shutter speed, and aperture, and learn the basics without investing a lot of money into a camera system.
Keep in mind that even if you buy a standalone mirrorless/DSLR, you have to consider a purchase of a better lens/prime than the kit lens. They are not cheap.