This is not very common knowledge, but it is no longer recommended to press S or U before B for SysRq. The official documentation of sysrq has stopped recommending this practice, as it may be harmful to modern filesystems. Writing to a storage device while the kernel is in a bad state has the potential to cause corruption, and modern journaling filesystems like EXT4 and BTRFS are designed to survive crashes like this with minimal (or no) corruption. Instead, you’ll likely want to use Alt+SysRq+REIB (and make sure you are waiting multiple seconds between each keypress, as they do not complete instantly!).
You may instead try to kill the most memory intensive non-vital process with Alt+SysRq+RF, which may stop you from crashing to begin with (this works especially well for memory leaks). SysRq+F will invoke the oom (out of memory) killer, which will kill the most memory intensive non-vital process without causing a kernel panic.
If you need to restart, the most ideal situation is to enter a TTY and cleanly reboot, in which case you can do Alt+SysRq+R to grab control from the display manager, then Ctl+Alt+F3 or Ctl+Alt+F4 (I believe most distros have the first login session run on the TTY accessible from Ctl+Alt+F2) to switch to another TTY. You can then log in and do sudo systemctl reboot if your computer is still responsive. You may need to kill some processes before your system becomes responsive enough to log in on a TTY, which is where Alt+SysRq+F is useful, but in extreme situations it may require Alt+SysRq+EIB.
So a basic order of steps to try may look like:
Try Alt+SysRq+RF and wait a few seconds to see if your system starts responding. If not, you can either try it another time or two, or move on to 2.
See if you can switch to a TTY with Ctl+Alt+F3. If so, try to log in and sudo systemctl reboot. Else move onto 3.
If you are in a TTY, switch back to the main login with Ctl+Alt+F2. Then do Alt+SysRq+REIB.
In the spirit of other users giving mnemonic devices, you could remember REIB with Reboot Even If Broken, or the oom killer RF with Resolve Freeze (someone else can probably think of something better for RF; I’m not great at making mnemonic devices).
TL;DR: There are SysRq combinations that are less prone to damage/corruption than Alt+SysRq+REISUB, so use the above flowchart, or just remove the S and U for Alt+SysRq+REIB (if you don’t want to troubleshoot first) for less chance of filesystem corruption from a bad kernel. You can often recover the system without having to hard reset (Alt+SysRq+B). And ALWAYS wait between SysRq keys, as they do not finish instantly.