I’m not saying you can’t want the a different option – I’m just offering you ways to get what you want (or nearly what you want) now, instead of waiting for a feature update. This might require you to be flexible and use a less-than-exactly-perfect solution.
If you have the subdecks each set to the same limit, then you should use that for the “Preset” limit, and change just the parent deck to a “This deck” limit of the total (or unlimited). If all the decks are set to different limits … it seems like you’ve already volunteered for a certain amount of cumbersome limit-changing, so you probably already avoid changing them very often.
It seems like decks running out of cards won’t happen very often, but you can give your last deck a “catch all” limit, or you can insert intermediate parent/subdecks to help control the flow of cards.
[If you have so many decks that this is too difficult to manage, you should take a moment to consider that you might have too many decks.]
This may be one of those situations where you have to choose what your priority is – dynamic new card limits throttled by max review limits, or random deck sampling.
If you look at some of the many threads ZornHadNoChoice has linked to, you’ll find that “Reposition” is often suggested as a way to manage this:
[Which @Keks already said too, but I didn’t see it!]
You can use that in each deck to keep the cards in that order, but stretch them across the same series of New-queue positions (using different “step” changes in different decks, and not worrying about any overlaps), so that a New card gather of “Ascending position” will pull from the decks in an order akin to random.