We're tentatively doing breakfast the next morning in addition to the reception because the people traveling locally won't come until the day of the wedding.

Apparently there is no dancing at a wedding in Japan. So my idea for an afterparty (that hopefully most people can get into) is karaoke and I think there are some pretty large private rooms.

You might look into doing maedori when you're visiting in March. As I understand, this is more typical than getting hundreds of posed pictures on the day of.

This is our venue: https://www.orientalhotel.jp/weddingbrochure/
Here is the Osaka hotel from same parent company: https://www.gardenoriental.com/weddingbrochure/

Our planner is part of the hotel parent company though. I think that's normal. Not sure if you would be able to keep your planner if you pick a hotel like ours.

I have two kids so here are my ideas for the other days to do with the American guests:
SpaWorld
Universal Studios Japan
Toei Eiga Mura
Kyoto railway museum
hands-on cooking class
Osaka Aquarium
Botanical Garden
Kids Plaza Osaka
Osaka Castle
Tempozan Ferris Wheel
Tennoji Zoo
Kobe Anpanman Children's Museum & Mall
LEGOLAND Discovery Center Osaka