Earlier quoted context omitted.
You can fix the problematic service with: systemctl edit service-name [Service] TimeoutStopSec=XX You can also override the global default DefaultTimeoutStopSec in /etc/systemd/system.conf.
See, this right here is the (technical, not social) problem with systemd. Whether anyone points out that systemd is doing something stupid and wrong, the systemd shills' answer is always "well duh what did you expect when you didn't modify one of the hundred default configuration files to have `[stupidshit] = false`! You brought this on yourself!" And you're lucky if that even works to solve the problem.
Summarily killing processes with a SIGKILL should really be a last resort because it can very easily corrupt data.
If you have processes which just hang for ages during shutdown, there is probably something wrong with your system (or the process's code) which might warrant further investigation.