If you know some underrated mangas/manhwas and gems hidden under piles of rubbish, pls recommend them to me! I've been deeply sucked inside the Historical Mangas blackhole!

Something similar to 'Song of the Long March' would be preferable. It can also be set in medieval period that'd be fine for as long as there is enough (reasonable) dramas involved and a (reallyyy) slow-paced romance which would make me crave for more. 😄 THANK YOU IN ADVANCE!

I recommend "Home" drawn by UEDA Rinko. It is a great read.

I can recommend
Antique Romance
Barairo no Yakusoku
Kusuriya no Hitorigoto
Bameur Geonneun Seonbi (historical + fantasy)
Saiunkoku Monogatari

If you're looking for an epic female mc, mangas that are similar to Song of the Long March but not strictly "historical", I can definitely recommend
Akatsuki no Yona
Basara
Anatolia Story

other than these, among historical mangas I just have to mention Golden Kamui that doesn't have a great romance but is very informative and original, and Innocent (SAKAMOTO Shinichi) which is once again very original.

Otoyomegatari
Kaze Hikaru
Emma
Drops of Dreams, The Golden Birdcage
Black Winter
Seirou Opera
Yokohama Monogatari

These are technically alternate history or isekai/time travel, but they're very well done historical romance:
Red River
Namida Ame to Serenade
Ooku

A lot of good ones already mentioned (Namidaame to Serenade is one of my favorites right now). But I have a few more recommendations.

The Moon That Rises in the Day - This one is part historical romance and part modern day thriller.

These are both less epic early twentieth century manga with nice slow romances:

Palace Meidi

Usotoki Rhetoric

orpheus no mado
tenma no ketsuzoku
red river
saiunkoku monogatari
gu fang bu zi shang

ꉂꉂ(ᵔᗜᵔ*)笑