> Is fedora / arch / etc really more stable than ubuntu?
Can't give you a definite answer on that, I am quite happy with Fedora but only using ZFS on my almalinux and freebsd system and my only experience with Ubuntu in recent years were LTS releases on servers and that was pre v20.
I remember that the kernel changed something a few years ago that broke openzfs and it took time to get back support on some distros so I would be afraid the kernel break something and force me to avoid kernel security upgrades while it is worked on if I was relying on it on a Fedora.
I believe running nvidia hardware and ZFS, or anything that is not using a driver included in the mainline kernel, is unsuitable for a distro running fairly recent kernels if you care about potential breakage. That is what LTS distro are for.
One alternative to super bleeding edge distro such as Arch is to use something like opensuse slowroll which is a kind of hybrid model between standard and rolling release. Every month or so it gets an upgrade to the lastest snapshot from the OpenSuse Tumbleweed rolling release and between each snapshot it only gets security updates correcting major bugs or CVEs. I only have used it for a few weeks in a VM but I guess if a major breakage would impact tumbleweed you would continue getting security updates until tumbleweed is fixed. But I have no idea if OpenSuse is an openzfs friendly shop, I think they are full on btrfs adoption instead.
> If I use a dinosaur, I'm worried I'll then need to work around its issues - can I easily install modern lldb on an old distro with an ancient glibc? I've tried that before, and it was a huge headache.
When I say a dinosaur, I was exagerating. For example all the toolchains and software you mention in your posts are perfectly usable on a long term distro such as almalinux 9. I can't speak about nvidia because I don't have any machine with it but Almalinux is a better proposal for openzfs. Nodejs is easily installed through nvm, I am using the most recent firefox, librewolf and ungoogled chrommum from the flatpaks, etc. It is not using an old glibc.
Apparently lldb is part of almalinux 9 but I have no idea if the version is modern enough for your use case:
https://almalinux.pkgs.org/9/almalinux-appstream-x86_64/lldb...
But as said before, you are just 2 commands away from a more recent Fedora toolbox available at your keyboard fingertips [1]:
$ toolbox create --distro fedora --release 40
$ toolbox enter fedora-toolbox-40
[1] and distrobox is available as a package on EPEL repos so you would have an arch or ubuntu available easily the same way.