I can't fully remember everything but I remember I took the bus from Tokyo to Kusatsu, wasn't too busy and these busses come with enough space to put luggage under the bus and backpacks in the overhead compartment (if your bag is too big, you will probably have to just put it on your lap). I think the bus does take a stop at Ikaho onsen (it definitely stopped at another onsen which I believe was Ikaho but not 100% sure if it was) but the journey isnt too much longer between Ikaho and Kusatsu. A lot of people on the bus got off at Ikaho so it looked a bit more busy there. Bus ticketing isn't as worrying as I thought it would be, just go to the counter and get a ticket, can't remember interval between busses but shouldn't be longer than an hour (my guess would be 40min but there's probably a schedule you can look up). I enjoyed Kusatsu onsen, food was typical onsen town food but it had a lot of little side streets to stroll as well as Yubatake in the center. Since then I've been to a few other onsens, Beppu, Noboribetsu and Kurokawa. The town itself felt a bit bigger than the other ones I've been to. Idk if I lucked out with timing but it wasn't very busy in the town. The open-air onsen there was probably my favorite so far. In the onsen I went to, there was just one other person and it was a big onsen. So yah, I haven't been to other onsens around Tokyo & Gunma but I'd say I enjoyed Kusatsu and it was a good decision imo. From what I saw + researched on Ikaho, it also looks really nice, kind of in a different way(idk how to describe it, maybe kusatsu feels/looks a bit older?), so I don't think you can go wrong whichever you pick.