The fact that you can "assume Vulkan exists" helps a lot (both hardware and software renderers exist). Do remember--Wayland predates Vulkan by almost a full decade.
In addition, you can offload OpenGL compatibility to Zink (again leaning into Vulkan).
> pull in a group of long term maintainers.
"Use new cool language" seems to be a prerequisite for this nowadays ...
At least Zig is very compatible with C.
You can't "assume Vulkan exists". Any pre-2016 hardware won't have proper hardware support for Vulkan and that's a lot of hardware still in use. Software renderers are unworthy of any serious consideration due to the perfomance drawbacks.
Just use OpenGL. I don't know when this trend to overcomplicate everything using Vulkan began, but I hate it.