What works best for gaming nowadays?

Depends on your system/setup. Though, you should be able to get it to work on most distros (eventually). Consider checking out https://linux-hardware.org/ to see which distros are known to work for your new laptop. Personally, I'm inclined to believe that Bazzite is the most hands-free solution for most people. However, other good options include Linux Mint and Pop!_OS.

Is manjaro good for that?

I really wonder where newcomers get to know about Manjaro 😜. Manjaro does a bunch of good stuff and has in the past been great for providing a platform by which Arch Linux (Manjaro is based on Arch) could be (relatively) easily consumed. However, Manjaro has since tarnished its own branding (see Manjarno). Hence, I find it hard to recommend it ever since.

I’ll also do some book keeping, writing et cetera but I don’t think it’s much to worry about.

You should be fine unless you need specific software (that's not available on Linux) to do those things on. For most software there are decent alternatives; https://alternativeto.net/ might be a helpful resource in that regard.

I also hope to use my Valve Index on it.

Unfortunately, I don't have any experiences on that. Therefore I abstain from giving any further comments. Wish ya good luck with that though!