Delay loading CSS is a powerful optimization technique. Some build systems create a minimal critical inline CSS and load the rest of the styles after the initial load. This ensures a fast initial paint and no CLS as the critical inline CSS gives the page structure.
I also advise against it as it can go terribly wrong if you do not know what you are doing.
rel=preload tells the browser that the respective resource needs to be loaded fast. It’s a hint. In this case, rel=preload is the exact opposite of async or defer.
As style sheets placed in the are early discovered by parsers and will load with a high priority I do not usually preload them, I let the browser do its own thing.
If you want to optimize your theme you need to know the basics, if not just go with the theme give you. Otherwise, you risk making a mess.
Most themes from the app store are well optimized.