Hello, I've recently got a used TI-84 Plus CE with the 5.2.2.0043 firmware installed. Which version should I install that allows me to get max performance while retaining ASM support?

5.4.0 is a good compromise between features/bugfixes and ASM support. It's 5.5 that drops ASM officially.

Does it have the Asm84CEPrgm token?

It does (technically they all do IIRC) but not directly choosable from the catalog. Not that you'd ever need it unless you like writing your own asm programs on calc in hexadecimal. TI's official workaround for that is that if you need it, just transfer it into a program from ti connect.

But if you really need it handy, get 5.3.1 instead I guess

Is it possible to downgrade if something goes wrong?

or just keep a prgm named ASMTOK that stores all of the asm tokens you'll ever need in archive and use cesium to open it and copy a token when you need it

5.3.0 makes it so you do not have to use Asm() to run ASM programs, which is nice. It offers the best compromise if you do want to write programs using Asm84CEPrgm on the calculator.

If you use 5.4.0, while you can recall the Asm84CEPrgm token from another program as suggested, it is annoying to run such "unsquished" (user-readible hex code) ASM programs: on 5.3.0 & below, you can just run the program directly, but on 5.4.0, you must first use AsmComp() on it, & the output has to not exist. So if you are testing things, this means having to go delete the squished program (the output of AsmComp()) each time you want to change the code.

On the other hand (& as Adriweb already said), this is not an issue if you are not writing hex codes on the calculator.

Is it possible to downgrade if something goes wrong?

If you have a very old CE (hardware revision < C), yes, but otherwise it's best to assume "no".
But why would anything go wrong, especially on pre-5.5 version...?
You'll be fine lol

If you want to program Asm on-calc (I recommend not doing that until you at least know the basics using a real assembler), whether or not you upgrade I'd definitely use ez80 Studio or maybe Basm. The OS Asm programming stuff kind of sucks.
And honestly, if you're going to install a shell like Cesium or CEashell anyway, I'd just upgrade to a good OS. With CEashell you don't even notice the normal OS since it lets you run Asm programs from the prgm menu.
If you're dead-set on not upgrading, still install CEashell and ez80 Studio. They're great utilities.