The obvious one would be to fine Apple until they implement Android-style mechanisms for openness. As pointed out above, although they're being lumped together, Android is leagues more open than iOS. Does it even make sense to talk about an Android "stranglehold" when the OS is forkable and many vendors have their own spins on it, with new features often appearing there first before Google even have it themselves?

In reality they are going to be treated together, because governments tend to reason in terms of countries and people not technical details. Apple/Android are both rich US tech firms who aren't going to leave the market regardless of how big the fines get, and in Android's case literally can't (because someone else could simply license it or use it and step in to replace Google). So it'll be like all other attempts at tech regulation, it'll just turn into cranking the handle on the cash cow to try and fill a part of the lockdown-created budget holes.

Requiring them to stop banning competing browser engines? That's the number one remedy considered by the CMA.

You should read the study, it's very insightful.

Apple has budget to the requirement for allowing alternative payment methods in dating apps for the Dutch market following a lawsuit and racking up a noncompliance fine they'll now need to pay anyway.

These regulatory bodies can and often do have an effect on large corporations if the punishment for ignoring them is set high enough.

Progressively larger fines?

fill their pockets and then shut up for a few years