https://plantuml.com/ is free and nice. Plays well with Markdown and C4 diagrams. The only UML-specific diagram type I use is the sequence diagram but that is very useful IMHO.
It can be a bit clunky setting up the jar (or hosting it yourself), but it's still the best worst option. It scales pretty well and can draw some very complex flows, while still giving you the best in just plain ole text editing.
I could have never maintained the diagrams I used for other developers on my teams or folks on the audit/regulation side of thing without it.
The default styles are really unprofessional looking and I think a lot of folks look the other way once they see that. C4 diagraming with PlantUML is also a breeze for systems diagrams.