Issues with by MBP (M1) are almost always memory related. The Activity Monitor is my friend to find out which processes are hogging precious memory across my seven virtual desktops.
My biggest win was with Firefox (with lots of windows and tabs open to allow quick context switching between projects) when I discovered the "Auto Tab Discard" extension which frees up memory for inactive tabs.
My Activity Monitor Memory Pressure is mostly green now.
My MBP is over 10 years old and runs just fine.
Have you looked at the processes, there might be some service that has become a hog.
Another possibility is that you have written a lot files to your SSD and/or your filesystem is near full. Another issue might be excessive swapping. Once the wear levels or saturation on the SSD hit a certain threshold, they can become very slow.
What are the CPU/specs for this machine?
Switch to Asahi Linux, macOS is torture.
> screenshots take 3-4 seconds to arrive in their folder
You might be running into an intentional behavior change with screenshots in newer versions of macOS. When you take a screenshot now, by default it pops up a little thumbnail of the screenshot in the lower right of your screen that lets you edit the screenshot before it's written to disk. Even if you ignore that, the screenshot doesn't get written until the thumbnail goes away. You can disable this in Screenshot.app > Options > Show Floating Thumbnail, and then your screenshots should get written and show up immediately. Hopefully that's your issue and not just Finder failing to update.
> switching User accounts takes ages
MacOS has to wait for all your user services to stop and then start all the next user's services when switching, so it might be worth looking into what services are attached to your users and trimming them down via launchctl or something.
I'm typing this on a 12 year old macbook that's still perfectly snappy, so hopefully you can breath some life into yours. 3 years certainly shouldn't be a death sentence.