This is a bit of a miss-match between ElasticSearch Essentials and XenForo Enhanced search.
ElasticSearch Essentials controls suggesting threads by per-forum per-user-group permissions, while XenForo Enhanced search does it via a single select menu.
I'd say the XFES method would be preferable for us, but if you think it's better to opt for it to be per-user group (we wouldn't really have a use for this in our case, but maybe others might) that's fine