I prefer not to get any messages at all when things go right
Do not display "things did go right" level warning then. Select your floor with the --priority=[0-7] switch, where 0 refers to emergencies and levels 5,6,7 are notice, info, debug messages, generally not indicating problems.
You can also chose standard time format and ignore non-system entries - or a combination of all three:
journalctl -o short-iso-precise --system -p 4
Where this is still insufficient, pipe that through an appropriate grep -Evf ignore.regex call, and please file bug reports with your distribution whenever you run into messages with unjustified warning/error level.
These messages are useless and pollute my log
These are not even the most useless in your logs. These are properly timestamped, clearly attributed and legible. They are going to help you when you inevitable run into one of the actually useless ones - when you are trying to diagnose something and need context.
My apps are perfectly well behaved. They only log what they need to log and what I want to see.
This aligns well with the notion of assigning levels to log entries of varying importance. In this case, all their logs shall be treated as high-level. To express that in their respective unit files, use SyslogLevel=3.
I don't believe systemd is even capable of performing
I don't think performance is what gives Microsoft this sort of market dominance.
