I went to Senjojiki Cirque in the summer.
In the summer, lines form for buses and ropeways, but it's still winter in April, so I don't think it will be too crowded.
Take the bus from Komagane for about 40 minutes, then take the ropeway for about 8 minutes.
According to the route search on Google map, it takes about 2 hours to 2.5 hours one way from Matsumoto to Komagane by train.
In the summer, it took about an hour to walk the hiking route and about 15 minutes to rest at the hotel cafe.
The hiking route is probably not walkable, so your stay may be shorter.
The ski resort opens in mid-April, so it will take longer if you want to ski.
Lifts are not available, so you'll have to walk yourself up to the slide.
https://www.chuo-alps.com/hotel/ski/By the way, since it was over 2600m, I had mild symptoms of altitude sickness.
If you are not accustomed to high places, it may be better to research measures against altitude sickness in advance.
It's not as high as Mt. Fuji, so I didn't have any symptoms up there.
If you go down the mountain on the ropeway, the symptoms will subside.
On April 15th, it may be difficult to see by Lake Suwa.
Komagane is famous for sauce katsudon, so please try it if you are interested.
http://www.kankou-komagane.com/spot/inspection1.php?c_id=2&cj_id=5Things to do in Komagane
http://www.kankou-komagane.com/spot/inspection1.php?c_id=1&cj_id=1