So you are generating 3D tree LOD. Then the nearest LOD trees use 3D LOD models and usually not billboards but the 3D tree LOD nif as can be seen from the DynDOLOD log.
The brightness setting for tree LOD only applies to billboards. The 3D LOD models usually do not need a brightness adjustment, because their 3D leafs have shaders and normalmaps and should react similar to lighting like the full model trees.
With a recent update some of the leafs LOD textures are not being put on the texture atlas (will be fixed again with next DynDOLOD version). Instead the LOD textures are used directly as is.
It turns out the LOD textures for the Apsen 3D LOD trees in EVT have really bad mipmaps. Either remove the mipmaps (with gimp) from the Textures\lod\treeaspenbranchcomplod0*.dds or replace them with the ones from Textures\lod\rat\treeaspenbranchcomp0*lod.dds in case the match the color tone (depends on which Aspen trees version you installed)
TexGen has no effect on the tree LOD leafs or the billboard trunks used by the hybrid 3D LOD trees.
I hear you. Both methods you described worked perfectly. Replacing the textures makes the colors of the LODs not 100% right and removing the mipmaps is obviously causing flickering once I am approaching the trees. I removed the mipmaps and recreated them by myself:
Works like a charm. The flickering turned down significantly and some LODs are a little too bright in my taste but this is just very nitpicky and when I am running around and focusing on other things, I will probably never notice it.
I can not thank you enough for your help. You were doing me a huge favour and I really appreciate that you took your free time to solve my problem.