With 6 teams so far, we still have 5(?) team spaces and not really any interest currently.
Is it worth creating replaceable Manager-driven B-Teams? Force them to use a poor tyre and a low-roll Tier 3 engine and have team managers drive them. The drivers can club together to work out a spending strategy for those teams where needed.
A bit like Team Keranen in Season 5 which was created out of necessity more than anything. If a new manager comes along mid-season and wants to join the game, they can take over one of these teams, including whatever money it has earned and points scored, but crucially they can then sign new contracts with tyre and engine companies if wanted.