Any suggestions re Kusatsu travel dilemma?

My friend and I are going to be traveling around Japan in May and have been invited to participate in a gathering in Kusatsu Onsen. We are trying to fit it in to an itinerary and are wondering what the most efficient ways are to make the trips detailed below. I have added some notes re: what options we've found thus far and would appreciate ideas if someone knows a better way to handle things. I looks like daytime travel from Kusatsu to Kyoto will take all or most of a day which means we will likely have to skip visiting one or the other since we don't have a lot of time to spare. Thank you!

Kawaguchiko to Kusatsu Onsen:
Bus or train from Kawaguchiko to Tokyo
Train from Tokyo to Takasaki to Naganohara-Kusatsuguchi
Bus from Naganohara-Kusatsuguchi to Kusatsu Onsen

Kusatsu Onsen to Kyoto (overnight version):
Drive (?) from Kusatsu to Tobu Yunomaru
Night bus from Tobu Yunomaru to Kyoto Station

Does it have to be Kusatsu? There are way closer onsen better located between those cities.
e.g. check https://www.amazon.com/Nagano-Onsen-Guide-Day-Use-Springs/dp/153719120...

so maybe Gero or Onsen arount Suwa are more convenient?

Ah, sorry, overread the part with the gathering.

With the exception of the bus ride, The Tokyo Wide pass covers train transport to Kusatsu Onsen.

http://www.japan-guide.com/e/e2361_06.html

I would think that it would be far easier to travel from Tokyo than from Kyoto. Since all of the travel shown on the map of the Tokyo Wide Pass is in either tan or red, you should be able to also make the journey with an active JR Pass.

I would suggest that it be a day trip from Tokyo, or, a stop while traveling from either Kanazawa or Nagano to Tokyo.

Good Luck, in planning your trip.

Thank you both for replying to this post. It's helpful to see your suggestions and also have confirmation we aren't misinterpreting transit maps or otherwise missing a big piece of the puzzle.

We're pouring over maps again and will post again if we have a pressing question.

Thanks and have a great day!