ELO still can’t be used because of the player count but we could have another ranking system. Something that doesn’t influence who you play(yet) but maybe a tier based reward system.
The lower ELO/ rank of the opposition, the less points you gain for a win and the more you lose for a loss, towards your next rank (standard ranking)
Rating based off k/d and score could lead too intentionally prolonging matchs to boost. Basing it solely off a win reward for the winning team and a skill judgement for the losers would work better.