I've been looking at draw.io and PlantUML - what has worked well for others? draw.io is much easier to get into, but I can see declarative diagrams making maintenance easier for long-lived docs.
Being a person who doesn't typically want to draw diagrams, I opted to go with draw.io as it looked very simple and easy. However, I was nudged to try PlantUML and to my surprise it was a lot simpler and faster to create diagrams. A point which you mentioned is maintaining the diagrams for long-term. We have a docs folder in our repository which includes all our PlantUML. It's quite easy to pick up as a language, and because it generates the diagrams for you, you don't waste minutes fiddling with getting the arrows all matched up. I would definitely give it a try - makes life so much easier!
Plus, there is a VS Code extension for it as well, which auto generates the diagrams when you save.
After having used both Draw.io and PlantUML, I prefer the latter. Far easier to get consistent drawings from the whole team without fiddling. And updating the drawings later is also easier because the language is much better than Draw.io's XML.
Plus, there is a VS Code extension for it as well, which auto generates the diagrams when you save.