I agree its a complicated ask, perhaps theres cause for a 2nd on-field referee like in the NRL, make one referee the senior referee and one the junior referee, in general play the senior referee primary focus is on the ruck/scrum/line-outs/maul, the junior referee is watching the onside line and both can rule on foul play.
More broadly the referees should be encouraged to communicate before penalising, they should be verbalising and telling the teams to stay online or identifying individual players who need to take a step back or pull out of the play, this would serve as a warning for players and those who don't comply are obviously penalised.
With this you would then remove the TMO or at least mitigate the need to use it so extensively, hopefully a system of verbalising the referees intent would an extra referee doesn't just lead to over-officiating.