Hey! A finnish trans masc here who loves to go sauna! And happens to not had a top surgery. I think that there should be no problem of you depicting a trans person bathing with another person, especially if its someone close to them. I understand that the cultular differences wont translate immediately to someone who is not familiar with sauna culture (both now and in history) , but i still think you should do it. It would really amazing to see.
sauna is always such an interesting cultural concept to discuss, by which i mean i won’t be doing that now because i would have a whole philosophy essay draft ready. the idea is definitely something i want to explore, perhaps in a comic form.