I've worked remotely for 4 years now. We try extremely hard to avoid talking in private chats. Each person has their own channel which we try to use over DMs - this works really well although a few people seem to default to DM.
I really miss real whiteboards.
The only thing worse that being on a software team that has no whiteboards, is being the only remote person on a team that has whiteboards in the office.
You can get an entry-level Wacom for < $100, and it scratches the whiteboarding itch for me in a lot of situations.
I’m also not convinced that there is a tool that when mastered can provide the same fluidity, but I can claim that, and to the contrary I’d love a recommendation.
It’s definitely not the same as drawing on a whiteboard, but I’ve found it useful for explaining abstractions.
(That said, if whiteboards are important to a team, I would imagine there's software for sharing tablet drawing. I've heard there is. I just haven't investigated.)