Open source Miro alternatives?

Does anyone know a really good replacement for Miro? I'm looking for something where you can have as many contributors as you want working on one board and share those boards with other people and share visuals from those boards with other people. I've never found something that looks professional, that has high usability and the features of Miro. I've tried Excalidraw, but it looks kind of hand‑drawn, and I'm not looking for that.