You say Teams racing at home Mondays and Thursdays mean they could have a GP Rider.
Ok that’s a true statement. It also means they would be available to race at other Mon/Thurs tracks currently five.
Then you add “a current championship team would do anything possible to arrange fixtures to get the GP star there”. However, For the large part those days could only be Mon or Thurs as the star would not be available or willing to undergo a late and chancy travel schedule. Remember riders who race in Poland are required to be there the day before a meeting not just race day.
It is highly unlikely that a guest would be available to equal the level of the GP rider (they will have their own fixtures) and let’s not forget this year as an example Leicester would have needed three guests - just not workable.
However when all said and done, one league, seven man squads equal eight teams. Nothing more but maybe something less.