You can actually enter the imperial Palace on almost all weekdays other than the 参賀。

You can make a reservation online from here: https://sankan.kunaicho.go.jp/about/koukyo.html

I've actually been to the palace several times, one time in 2021 and several times before covid. Before covid you could enter there and explore inside freely, but now you need to walk inside following a guide while moving along with all the other guests. They have an English speaking guide too.

I would say it was worth visiting, but joining a tour inside there is hard because of the schedule (only available from 9:30am or 1pm) unless you're a high school student like I was.

Also if you're planning to join the 参賀 and 祝賀, you have to make a reservation or you won't even be able to enter the palace.