So is there more or less clear guideline about how to get bigger breasts? Because everything Ive read so far is either way to random (for example, that pool sting thing) or just never happened with me at all (magic wand, how the hell I even get that?)
There are certain ways to get them, but RNG is enforced in the game. some events simply won´t fire in one playthrough, only another one. And some are gated by your behaviour, e.g. you will have a hard time getting events with May if you do not investigate the ressort often.
The magic wand has been changed. You can find it during room-cleaning events and you have to search for it without getting caught. then you have to experiment with it. I personally leave it be for the moment, not really interesting enough, if you know and want to reach specific other events.
Easier way to get some changes is getting on the good side of Dr. Katsumoto. For that, you have to a)investigate the ressort often, b) learn the access card trick (normally during kitchen duty), c)then sometime during investigating the ressort, a drunk will come out of room and wants help, when this happens help him to the toilet and take his access card activate it and go into the room he came out of.
Follow the event and next time you go to sleep, you will have some "talk" with important figures of the ressort. Agree to work for Dr. Katsumoto and within a few days she will call for you. Among this event she will ask you if you want "improvements" either towards feminine or masculine side. You can then choose what you want.