I've found it very difficult to make screensharing work at all in Wayland.

On the other hand, the reason I switched to Wayland was that I wasn't able to get multiple monitors with different pixel densities working in Xorg. That works fine in Wayland, and since one of my monitors was unusable without being able to choose different pixel densities for each monitor, Wayland it is.

If it matters, I'm using Pop OS on a System76 Thelio.

I don't use Slack for that, but it works on Zoom, Teams, and Meet. So I suspect it works.

I have set up fractional scaling of different fractions on all three of my screens. No issues on Wayland.

The biggest issue with slack and wayland is due to Slack running via Xwayland (so not wayland native).

When electron 12 is released and slack will be build using it, it will be possible to use slack as a native wayland application and then screensharing should work.

One of the things I noticed with fractional scaling on my 4K monitor that video playback occasionally looks choppy, which is slightly annoying because I don't want integer scaling because my other monitor is a factor of about 1.4 off.

That never worked under X11 either so not a problem with wayland, just it would be nice to find a solution.

You could try it in chrome which has wayland support. Wayland support was added to chrome and electron recently but it looks like most electron apps are on old versions.