Travel from Kusatsu to Koyasan (Mt Koya)

We are proposing to travel from Kusatsu to Koyasan (Mt Koya) in one day (about 8 hours travel). It appears to involve going back to Tokyo before heading to Osaka and then onto Mt Koya. Is this too ambitious? Is there a better way? Have been using the online rail travel planner.

It is more like 9 hours. Could be even closer to 10 hours if you travel by Japan Rail Pass. There is no faster alternative than going via Tokyo and Osaka. Travel via Karuizawa can be faster than via Naganohara-Kusatsuguchi, but the bus between Kusatsu and Karuizawa s not covered by the Japan Rail Pass.

Is this too ambitious?

Rather than ambitious, I would consider it boring and a waste of almost a full day of your time in Japan.

Is there a better way?

I recommend to split the journey up, e.g. visiting Tokyo and Osaka along the way. Or you could travel via Nagano, e.g. stay overnight in the Kiso Valley or somewhere else along the way.