I think the easy solution here is to remove the word amateurs, allow clubs to pay money to players in whatever process they like, and if clubs play overs and struggle in the future that then becomes a governance issue for clubs
Just tier it - either teams have an amount for a year they can spend or a cap per week. If a club then in D grade wants to spend money to get players to help their club move up the ladder they can, but they have to balance this up with the players they could lose by bringing in new players.
It boils down to clubs then deciding what they want to do, they can choose between short term success or long-term players in the same grade