The performance setting for Eien Musubi on this month's Shounen Club Premium is very pretty
Seeing that this month's Shounen Club Premium centred on Kisumai being at Nagano's Karuizawa , that church which Kisumai performed at , it's the Karuizawa Kogen church
https://japantravel.navitime.com/en/...NTJtrv0535-en/
I doubt NHK Premium specially built a church for their performance LOL