I guess there's katakana Sa サ and Su ス, if that's an improvement.
I like the katakana idea, I wonder if I can train myself to recognize the Su one enough to start using that when I'm handwriting days of the week places
Monday - "понедельник", which is coming from the "day after the (previous) week", i.e. after the Sunday Tuesday - "вторник", true here, has "second" in the name Wednesday - "среда", has "middle" in the name Thursday - "четверг", also true, has "fourth" in the name Friday - "пятница", also true, has "fifth" in the name Saturday - "суббота", derived from the Hebrew "shabbat" Sunday - "воскресенье", almost the same word as "воскресение", which is the Christian Church word for the Resurrection (of Jesus Christ)