Transportation to Kusatsu Onsen

I intend to do a day visit to Kusatsu Onsen from tokyo. Based on what I looked up online, way to get there is to take a train to Naganoharakusatsuguchi Station then transfer to the bus to reach the onsen. Is there any bus schedule available online? Also is there a JR ticket office at Naganoharakusatsuguchi Station? I tried to look up the info online but could not find it. I want to stay till evening to see the illumination. Not sure what time the ilIumination starts so might stay and leave later. Hoping to get a ticket for shinkansen at the station when I am going back to tokyo.

Is there any bus schedule available online?

They depart pretty much timed with train arrivals:
https://www.jrbuskanto.co.jp/bus_etc/cntimep01.cfm?pa=1&pb=1&pc=j04501...

Also is there a JR ticket office at Naganoharakusatsuguchi Station?

There seems to be no proper ticket office, but a ticket machine which allows customers to talk with an operator. There is also a regular ticket machine that allows for purchasing shinkansen tickets. Furthermore, it seems possible to buy JR train tickets at a ticket counter in the Kusatsu Onsen bus terminal.

Last winter the illuminations were held from November 19 to February 14 from 17:00 to 23:00.

You need 4000 yen ,192 km, 240minutes
JR highway bus kanto will go from Tokyo to kusatsu onsen bus terminal directly.

Another way to get there if you have the JR pass is shinkansen to Karuizawa (1hr) and then bus to Kusatsu onsen. The bus leaves from JR station at Karuizawa.