@Harvey just let me know that there's a workaround for the VirtualBox issue that I've mentioned here. Since there may be some of you here who are affected by it, and may be avoiding Wayland (as I was) because of it, I thought I'd mention the workaround here instead of just in the Dev Tracker.
It turns out that the issue I reported is an old one, going back 3+ years. It's probably never reached the top of the priority list because the workaround exists. To use a virtual machine at full-screen on a Wayland system, all you have to do is turn OFF the mini-toolbar in the UI settings for each VM. Like so:
A side-effect is that the Latte dock, if you're using one, is also unavailable with this setting turned off, but missing both the mini-toolbar and the Latte dock is a small price to pay for the ability to use VMs in full-screen mode in a Wayland session. The phrase, "life-changing" came to mind for me, but that may be a little too much.