Thanks for the suggestions !

I've tried DungeonForger and it's indeed very simple and fits most of the requirements. But it's also old/dead and buggy.
I also tried ViewingDale, spent a couple of minutes with without being able to draw a single wall, so it doesn't pass my dummy test
Having to click on the small bullet in every menu to make a new menu appear was just painfull.

I also tried Gimp and its grid + snap functionality. Doesn't work as well as DungeonForge, but it's decent it doesn't crash
I will try to see what Dia has to offer

Thanks