It's great idea but I doubt that NDA/license will allow you to publish some sources and info from their repository in 'limited open source' project as GLB. For example RayLib unofficially supports Switch, but it requires to cooperate with someone (don't remember know who atm, most likely author of RayLib- RaySun). Maybe this would be better option (than Unity) for your Spectrum Next projects.
From what I know Switch uses proprietary Nvidia API similar/based on OpenGL/Vulkan - called NVN, most likely it comes with it's own license, there are some reverse engineered open source projects that gives some idea how it works. But due the nature of the device itself, as it's a console, it will be more targeted to certain things.
On other hand there is a port of SDL for Switch, but I'm not sure if it could be used for commercial games, if so, that would be easier way for GLB to support that platform.