Mewkit:
Yes we love sageuks ^^
I'll say Scarlet heart, we both liked Arthdal, Mr Sunshine....we have the same taste, she's just not use to chinese yet
I'm probably not the best person to recommend Chinese historicals, because I haven't watched that many, but I'll do my best ...
Similar to Scarlet Heart ... well there is the Chinese version (I haven't watched it). Maybe The Rise of Phoenixes, because it also has a really compelling, ruthless male lead as well as a power struggle between princes. However, The Rise of Phoenixes is MUCH more political than Scarlet Heart. Nirvana in Fire also has a power struggle between princes, but it seems to have very little romance and is nearly 100% politics (I'm only on episode 10, though, hopefully someone will correct me if I'm wrong).
Similar to Arthdal ... nothing is really similar to Arthdal. It's pretty unique among sageuks too.
Similar to Mr. Sunshine ... I'm really not sure. Maybe Winter Begonia. It's not a romance, but it does have a "period drama" feel like Mr. Sushine and it's also very beautiful and set in the early 20th century (I've only watched 1 episode so far, though).
Hopefully someone else will have some better ideas than I do ...