> there is no such thing as "mainstream Linux"

Yes, there is: https://linux.org (aka mainline Linux). You can have various DEs on top of it, but the main functionality depends on the drivers in the mainline Linux, not so much on the DEs. Most of the operating systems for Pinephone listed in your link have more or less the same problems and features, because they all use the same kernel. They all depend on the upstreamed support of the device.