How familiar are you w/ The Everly Bros?
Since you've recently upgraded The Beach Boys to 5 star status (I agree w/ most of what you say about them btw except you definitely like "Sunflower" more than I do) and are getting into 50's rock. The Everlys are my favorite artists from the 50's rock era and the obvious connection to The Beach Boys are their harmonies- each group is the only serious contender against the other in the harmony department and in both cases, the fact that the singers shared DNA is obviously a big part of the reason for that. The Beach Boys had more singers and Brian was a more advanced songwriter than anyone writing for The Everlys so their harmonies but Don and Phil's voices seemed to mirror one another more closely than those of any of The Beach Boys' singers.
Anyway, if you're unfamiliar, I definitely recommend the two comps below. First one includes all their early Cadence songs (their most famous/beloved material) and the 2nd includes later stuff for WB which is really good and unfortunately slept on pretty often. Simply put, pop melodies and harmonies don't get much better than this. https://rateyourmusic.com/release/comp/the_everly_brothers/the_complete_cadence_recordings_1957_1960/ https://rateyourmusic.com/release/comp/the_everly_brothers/walk_right_back__the_everly_brothers_on_warner_bros__1960_to_1969/