Your trials around the AS3935 piqued my interest a little and I thought that maybe the Blitzortung/LightningMaps people may have tried working with them. It turns out they have! The earlier generation Red board documentation [1] has a section on them and there was apparently internal-to-operators discussion [2] which isn't directly public, but which points to wxforum.net where there were quite a few results with at least one [3] detailing struggles to get it working.
This probably isn't directly helpful but I found it a fun trail to follow for a bit.
[1] https://www.blitzortung.org/Documents/TOA_Blitzortung_RED.pd...