Unable to adjust Space membership

I have a Business Hub deployment, v1.18. There are 7 seats, and the (“General”) Team has all 7 seats assigned.

Within a Space in the “General” team, I want to limit access to 2 of the 7 members - one Admin and one named member (who is one of the 7 seats).

However, the Private Space does not allow for the selection of only a few members. In order to do this, one of the 7 seats needs to be freed up, and only then can some members be selected. This, obviously, is disruptive.

Any suggestions on how to address this without temporarily removing someone’s seat?