Bulk add tracks to a group

I’d like a feature that, when you select multiple tracks (regardless of type, audio, sampler, instrument etc.) you can add them to a group (or effectively change their output).

You can already select multiple tracks in the bottom mixer to add to a new group channel (being created at the same time), but you cannot add to an already existing group channel

You certainly can. Select your tracks in the full mixconsole view (F3). Press Q-Link button. Select the Routing to your desired pre-existing group under the Routing rack on the mixer. Change it on one track and all the selected tracks will then change to the same entry. Deselect Q-Link when you are done.
You’re welcome!