Yes, please unpin the accepted answer. The answer accepted by the original poster is not always the best answer according to other users.
Unpinning will result in the answer that the majority believe is best will rise to the top, but the original poster's accepted answer will still be clearly visible as the one with the big green tick.
I agree with the commenter you cite, that your question on Main, in its current form, lacks focus because it looks for answers to multiple unmarked questions.
I do not think that asking for a text book recommendation instead will bring it on-topic, because that is likely to result in a list of each text book about projections and coordinate systems that ...
Since the question you cite involves PROJ, and:
PROJ is a generic coordinate transformation software that transforms
geospatial coordinates from one coordinate reference system (CRS) to
it is highly on-topic for GIS SE and, I think, less likely to attract suitable expertise to answer it at Stack Overflow.
I recommend that you delete it from there, ...