What are the forced memes (like "Triangle mate") for?
They are mate themes. If you have a better name or if you think they are just a subset of another better known theme, please do share.
You can read about triangle mate and some others on https://en.wikipedia.org/wiki/Checkmate_pattern#Triangle_mate
or google it and find many another chess sites with definition and examples.
Lichess users may have their own unique opinions on what counts as a specific mate motif. You can read about it in https://lichess.org/forum/lichess-feedback/so-many-puzzles-have-false-mate-motif