As the games best host can I make a suggestion.
A very simple toggle that allows you to turn promotion on and off between tiers. Now I hear you all shouting “that exists”. Yes but all I suggest is a tweak to this by adding/amending the toggles so there is two:
Promotion from rookie to Pro: on/off
Promotion from Pro to Elite: on/off
This allows promotion from rookie to pro to build up numbers there but allows league managers to then build up pro until such time as there is a decent number to allow competitive racing and also can avoid early promotion to elite and disengagement with the game.
Thank you and goodnight