I use Debian on an old Thinkpad and (mostly) don't have such issues. Installs and upgrades in particular work fine. I had probs with the wifi driver on my x220 but it works fine on the similar t520. Framework might be trying to do too much.

NAS shared folders

now that is something that can easily make the system unstable, especially a laptop that will disconnect from the network at least ince in a while. my experience is with KDE, that if there's an unresponsive SMB mount 8n the filesystem, the whole KDE plasma environment fill freeze left and right, maybe with the exception of the window manager. but I have experienced this with other programs too. I suspect they all do filesystem accesses on the main thread and that's why when a directory read hangs, they can't do anything even handle clicksuntil the read times out.

its infuriating honestly, in a sense. of course, I have got all my money back lol. but it's like nobody is testing software with SMB shares, but I guess probably same goes for NFS, SSHFS or anything remote

If it’s for work, I’d suggest using whatever works for you best. Sounds incredibly frustrating so I don’t know why’d you be so set on ditching windows. Use the tools that work for you. Having said that, I’ve been running Linux since early 0.99 kernels and Debian since 1.3 and stability is really unmatched these days.

Your screen flicker issues with browser sound like hardware acceleration related bugs and I’d hazard a quess that random freezes and reboots have something to do with graphics drivers as well. But of course it’s impossible to tell without logs, which you didn’t provide.

Another potential cause for random slowdown, errors and crashes could be overheating. Check that the fans are spinning and airflow is unobstructed. I don't remember from the top of my head and I'm not near a computer but maybe somebody else remembers his to check that all sensors are detected and operational.

Offhand anecdotes here as I only have Ubuntu on a 2012 MacBook.

That said the error post update is likely just a service that didn’t restart properly. Many of these are not necessarily critical, does it say what program crashed? A reboot would guarantee a fix here.

Unfortunately the issues with apps might be the snap packaging, this does slow apps down a bit which could cause pretty much all the remaining issues. I haven’t personally used it but might look up flatpak as a replacement and see if that helps. If others don’t explain how to do this I will try to come edit this later with an explainer or link or something to help.

I work on Linux and use Linux at home. I'll try to go through the problems you mentioned:

1. Just run the update command again in the GUI or terminal. If it doesn't work, we'll have to dig into apt with verbose logs but I haven't had apt break on me for over a decade unless I deleted something I shouldn't have.
2. Is Firefox installed as a snap/flatpak? That only happens with me occasionally when I installed flatpaks, they're just slower. Canonical can be a real arse about this stuff, they might switch packages to snaps without telling you and you might only come to know about it once you dig deeper.
3. All of these issues seem to related to your storage medium. Is the SSD OK? Open up the process monitor, sort by ascending order of disk writes/reads and open your applications one by one to see which one of them is the culprit.
4. Rebooting suddenly is not normal. Unfortunately, you'll have to go through logs for this one. Simple ones are dmesg and journalctl, we can dig deeper into them if you want to.

If I had my hands on your laptop I'd be running a vulnerability scan by now but I don't think the problem is serious enough to warrant it.

Id switch to mint, most windows like and all the knowledge youve learned will work on it. If you want true stability go Debian.

Great guidance here and I know you want to stick with Ubuntu, but but if you tire of trying to fix it try a different distro before you give up.

Lots of people swear by Ubuntu, but for others (like me) it's nothing but trouble. For instance, I get errors when running the latest version of Ubuntu on a current laptop that runs Debian 12 perfectly, and a previous Ubuntu load on one of our laptops (tried with a new SSD) had so many issues that I gave up and restored the Mint backup.

By contrast, we have 2 different laptops and one old desktop that run Linux Mint almost flawlessly. "Almost" means a system lock up every 3-4 months and the inability to wake from sleep for the desktop. Debian 12 was a bit more difficult to get fully working, but since the initial install it has been been completely stable with zero problems. We have one laptop that is running Windows 11 and it has more problems than any of the Linux machines.

Fixing problems is a great way to learn, but if it's not the way you want to spend your time you may be heading down the wrong path. Unless you have a hardware issue you should be able to find a distro that has few or none of the problems you've been fighting.

Just FYI. My family has used linux for 25 years on many systems and we do not have stability issues. We use mostly Ubuntu or Debian.

Have no idea why your having issues. Could distro or hardware related. Also are you sure your storage media is good.

Linux has always been the way you've described across many different distros for me over the years.

By far the most stable for me was Fedora. I've been running CachyOS over the last year or so and it's been solid.

Until today. For some reason KDE takes forever to startup now. A few apps have this problem as well.