Where to stay between Tokyo and Kusatsu Onsen?

Hi!! I am struggling to adjust the last part of the schedule of the last 2 weeks of my Japan trip. Me and my partner will arrive at Narita Airport towards the end of May, we will probably spend 1 night in Tokio, then we have 3 nights that we havent defined yet, then 2 nights in Kusatsu Onsen and finally back to 5 nights in Tokyo.

To consider that we are doing this part after osaka, kyoto, hiroshima and fukuoka, so we wanted to see more the country side but whitoit moving a lot, and still satying connected to an easy transport to Kusatsu

I would like some help or recomendations about the 3 nights before Kusatsu, I was thinking of Nagano since it has a better connection later, but I have read that as a city is not than interesting. Do you recomend doing Nagano as a central point and doing day trips? or do you have any other town or city in mind, maybe 2 nights at one town and 1 at another?