Personal experience Air Canada do this well, clearly marked at the gate and on boarding passes. Gate staff do enforce it.

As crazy as it sounds, I actually don't mind boarding last.

Quite often when my kids were toddlers, we would wait until the end, and let them run around as long as possible. Remember one particular time when flying back from PHX in WTP, we were the very last to board. My son & I got to our seat, he was 20 months so sitting on my lap, to see the look of horror on the guy's face in the seat next to us when we turned up. All turned out good, both my son and I slept 80% the whole way home and they guy congratulated us on our behaviour at the end! I laughed and reminded him, adults are often worse.

I do appreciate SH with a bag is a different game, but LH will wait to the end.