What can I say, it's cute and fluffy, and I love myself some BL, even if I'll probably never write about it myself.
The grammar is solid, no obvious or glaring issues that made my reading experience lessened, but there is definitely room for slight improvement, maybe the author can expand further and use more flavor-text to make all the "kun" stuff feel less forced.
I don't know, maybe it's just a preference thing with me, but it distracts me from the story. It works in anime, because it's spoken smoothly and naturally, but in text and stories typically sama would just be replaced with sir, or master. (In English, that is.)
Again, mostly preference stuff.
Definitely adding this story to my Library, as it's a very well made BL-romance/Historical Fiction. It doesn't feel cheap, it doesn't feel lazy. You can absolutely tell the author put their heart and soul into everything they've written, and that they earnestly care.
Well done.