Pedantic
Legend
To me the simplest way is to give such a legendary two spots in the initiative order, and have it go twice. No weird out of turn mechanics.
I generally think this is a much cleaner design, and it's something you can stack on top of an existing monster more easily. The risk mostly comes in a monster spending all those actions targeting the same PC and acting many times before the PCs get to react. That's significantly mitigated by giving the monster significantly different initiative bonuses for its two action slots, and frankly the currently structure of legendary actions doesn't really do anything for the picking on one PC problem.
Alternately, the game could just make an action economy comparison part of CR/EL calculations. If the system is upfront that mismatched numbers of opponents requires a significant gap in relative power and that such fights tend to be swingy as a result I don't particularly think that's a problem.