tbh i've always imagined that rather than a proposal they just kind of talked about it one day like "hey should we get married?" they'd still have a ceremony because they want to and saki especially would want to but i don't really see them proposing traditionally. if they did it'd be like ichika or honami i think. ichika does it accidentally while she's on one of her monologues about how she loves them and she just pulls out a ring box like these are for youuuu. and honami would just do it more normally like on one knee. not in public though because shiho wouldn't like that.