I recommend WBO : https://wbo.ophir.dev/

I am the main developer, and I have been maintaining and using it for several years now. Some of its advantages :

- It is fully open-source, free, and without advertisement. You can easily deploy it to your own server.

- All the whiteboards have an infinite size.

- It is fully web-based, you can use it without installing anything, even on relatively old browsers.

- It's translated in several languages.

- It's actively developed. In-development features include image upload, element resizing, new tools, and others...