I get the impression you've been enticed/subjected to some confusing technical jargon without noticing. Please allow me to shed some light.

Idk, there so much “mint and Debian are stable” but like… what’s the even mean?

Stable can mean a bunch of different stuff that may or may not be closely related. In the case of Debian, it's the name of its default release; the one in which packages are frozen for two years except for security patches. (Note that this naming scheme is not unique to Debian.) As such, a Debian installation will be unchanging for these two years, earning its stable designation (which, to be clear, just meaning unchanging in this context). Finally, this unchanging environment should provide a ton of stability (i.e. stuff just works), which is also referred to as stable. These three distinct meanings of stable are probably the ones you'll come across the most.

Is fedora 42 or bazzite going to be crashing regularly? Cuz… I doubt it?

The bold part is a clear demonstration that you understood stable to mean strictly robust; i.e. the third meaning discussed above. And to be clear, Fedora does a decent job at providing a reliable experience. (Bazzite even more so.) But not all three meanings of stable apply to it:

• For Fedora (and thus Bazzite by extension), the only stable repository is the one used to create its ISOs (i.e. the images used for installation). Beyond this, some packages are frozen within a release/version; e.g. you'll never get a major release update for GNOME unless you do a major release update for Fedora. But..., that's basically it; (almost) all other packages receive regular updates. As such, Fedora is often referred to as a semi-rolling release distro instead (as opposed to Debian being referred to as a stable release distro). So, to be clear, Fedora and Bazzite are NOT stable in this context.
• As (most of) its packages receive regular updates, it isn't unchanging either. And thus, NOT stable in this context as well.
• However, in terms of offering a robust/reliable experience, Fedora is pretty good. Bazzite is even better due to its atomicity^[That is, updates either happen successfully or not at all. So a random power outage (or otherwise) is not able to break the system's integrity.] and the superior distro-management allowed by the bootc model.

So, to answer your question, Fedora and Bazzite will not crash regularly. And, while Fedora might fall a little short of providing as robust of an experience as you might find on Debian and Linux Mint (assuming you won't FrankenDebian your installs), Bazzite may actually rival (and perhaps even eclipse/surpass) Debian and Linux Mint in this respect.

Seems easier to just go with fedora 42 or bazzite or whatever

For your purposes, I agree that going for the Bazzite-route seems to be the easiest.

but now idk what bazzite being immutable even means for what I can’t change and why that’s a big deal so idk.

This is a nuanced discussion that probably deserves more attention, but I'll keep it short for the sake of brevity. In Bazzite's case, strictly-speaking, immutability refers to how most of /usr's content isn't supposed to be changed deliberately by you. This is enforced by the system (in part) by making those files read-only.

In practice, though, there's very little you actually can't do with the system:

• AFAIK, you're forced to use GRUB; which AFAIK is the most used bootloader anyways. Here's a thread discussing support for systemd-boot.
• No support for UKI at the moment. But progress on this has been accelerated (relatively) recently; a presentation from just two weeks ago about the subject matter.
• The situation around dkms/akmods still ain't great and leaves a lot to be desired; you're basically limited to the ones found here. Thankfully, this is something else that's actively being worked on.
• While installing from Fedora's repository through bootc/rpm-ostree is possible, it's actually discouraged in Bazzite's case. Thankfully, there's a plethora of different means to the same end. Furthermore, sysext have come a long way and might become our go-to eventually.