Love of Thousand Years

Both dramas has the same FL. After her family the princess asked someone to change her face so she can avenge her family. She went through hardship from being a cleaner to a personal maid. She was favoured by the ML and SML. She was so determined to the end.

Both FLs are betrayed and return for revenge under the identity and face of another girl. During their revenge plot they get entangled with the ML who has secrets of his own.

Both dramas involve the FL having her face swapped with a different girl, and a ML who knows and loves her from her past lives/identity.

Both are historical dramas centered around the revenge plot by the FL who, after being betrayed and losing everyone she loves, returns for revenge through a fantasy twist (face swap vs. time travel).

Both dramas are historical and are about the FL who, after being betrayed and losing those she loves, has her face exchanged with another girl. She then returns for revenge. Both MLs have been in love with the FLs for a long time, something the FLs are unaware of.

Both FLs have their faces exchanged with another girl and are believed to be dead, and both are on a quest (to solve a mystery vs. revenge). Both MLs are mysterious figures who the FL initially doubts.

Both dramas involve a FL whose country/family was destroyed by the villain. She returns through a supernatural means (time travel vs. face swap/new identity) for revenge. Both couples have a love-hate relationship from the FL's side.

The actress are both a princess and goes into another setting where she becomes a servant to the male lead. The male lead is an immortal and falls in love with the female lead after undergoing tragic experiences.

Both drama have: soul-body swapping, martial arts, intrigues, plots, battles and in general they share a similar atmosphere.

Both Female lead have a bit similar story about Come back with different face and identity to revenge

Both leads have complicated identities which put them at odds with each other, and ML is in possession of something the FL needs that if she takes it will cost him his life. Both dramas have ambiguous endings with the leads within the world of a painting.

Strong FML
humble background (though she is a princess, after her family dies, she disguises herself as a maid)
ML hides his identity
ML is very powerful (immortal deity)
War strategies, palace politics

I don't know why this drama has a rating below 8 out of 10, but I really loved the love of the ML towards the FML. He waited for her 1000 years and saw her
going through reincarnation after reincarnation, but he never stopped waiting and watched over her.
If you haven't watched it yet, please do. It's highly recommended and I hope you would love it.