Is there an enthusiast website/community database of boards with clear documentation of Linux support, 3rd-party dependencies (patches needed, binary blobs), etc.?
Various projects keep status pages of their mainlining efforts. Example for Allwinner hardware from the linux-sunxi project:
https://linux-sunxi.org/Linux_mainlining_effort
Generally, SoCs that have lots of things marked "WIP" have community attention and are probably going to work at some point without kernel patches.