I’m not sure if any real decisions were made about Brethil but you posted a number of good pictures of beech forests:Right, Ladros should be set within Dorthonion. We're adding that particular site this season, as well as Tarn Aeluin. So, if anyone has suggestions for those, we can discuss. Dorthonion was set in the Altai Mountains of Mongolia in Season 4.
For Hithlum/Mithrim, we decided to use the Scottish Highlands in Season 3. So, Dor-lómin should either be a location in Scotland, or look similar to the Scottish Highlands to fit those adjacent locations. I don't think we get any detailed description of the Land of Echoes.
What did we decide about the Forest of Brethil?
Apparently (according to Encyclopaedia of Arda) Tolkien isn’t super clear if it’s beech or birch https://www.glyphweb.com/arda/b/brethil.phpBrethil apparently means 'silver birch', so I think this is an opportunity for a birch forest!