I disagree, the exceptions are not "few" and by making smart purchase decisions you can pretty much expect mainline support. You have to look at the trajectory of things and ongoing efforts.

For example my $15 Orange Pi PC (Allwinner H5 SoC) that I bought many years ago from AliExpress went from no mainline at all to excellent with only minor parts like the Crypto engine still requiring kernel patches.

This is thanks to the linux-sunxi, lima, and other efforts. For AMLogic there is linux-meson. For RockChip this is more spotty, but you can look at what Pine64 did in the past and their track record is actually pretty good.