Feature Request - Adding Synfonisk to a Zone

So I still have to manually select the group in the Sonos app after the Era100 started playing the line in audio?

I don't do this, but it seems like if you picked the Group Coordinator to play to the Group should stay intact.

I can make a zone with the Era 100 and the One and use them to autoplay my recordplayer. But I can’t add the IKEA Bookshelf to join them. These 3 speakers are in my diningroom and I have the Beam in my living room. I would like to autoplay to the diningroom.

The dining room is a physical room. Unlike Google Sonos does not adhere to a physical room as a concept, instead calling a (set) group of speakers a room - or a zone if you want them to really function as one speaker. So your dining room is made up from a room (the Symfonisk) and a zone (Era 100 and One). You could try to group the zone with the Symfonisk and Autoplay to either one of them. I am not sure if a zone can be part of a group though.

Zones can be grouped with other Sonos products in your system, and multiple zones can be part of a group.
 

https://support.sonos.com/en-gb/article/set-up-a-zone-for-multiple-sonos-products