I think Apple would develop a separate core (entirely separate SoC really) for such a move, so I don't think it'd be simply them pushing the clock speeds up. They like to take things slow though, and they've been working on kinda merging/porting some of their software (not unlike what they started doing leading up to them moving from Power to Intel/x86). I think it'd be a relatively slow transition though, where Apple will start by shifting the SoC that goes in the iPad Pros into the Macbook Air and Macbook (non Pros). The iPhone and regular iPads would share a SoC. Then probably a few years later we see them move the Macbook Pros and iMac to a third SoC.You're repeating a common opinion that if Apple (or anyone really) wanted to, they could scale up an ARM design and it would destroy everything. It's not that simple. Others can surely explain it better than I, but you can't just say that because it runs at x GHz and q voltage, it can run at y GHz at p voltage. So many different things affect frequency.
If there were massive gains to be had, someone would be doing it. Software is a problem, but less so every day. Apple surely has the money, but not all of the necessary technology. To answer your question, Zen+ at it's lowest p-state of 2.2GHz only needs about 0.75v on my 2600X. I think it's actually 0.775v but it bounces around so much because of background tasks it never stays there long.
I wish I could give you a better answer about high performance ARM and hopefully someone else can. You may find this article to be informative, though.
AMD shouldn't have similar delay, and I'd guess they won't have as much issues related to I/O stuff. I don't see why Zen 3 shouldn't be ready for Computex. As far as I know, TSMC's 7+ is on track, and already volume production - think Apple's SoC that is in the new iPhones is using it).Let's not forget the GloFo issues. AMD wisely didn't commit to them by the looks they were able to switch to TSMC. That could have bit them hard but I think it only cost them 1-2 months. Who really knows, though? I expect to see Zen 3 in late August at best to mid October at worst.
I could see AMD stretching things out more. Zen 2 Threadripper at CES (where it becomes the top gaming CPU by having largest L3 cache, highest clock speeds, and most power/thermal room). Where it launches shortly after. Probably Zen 2 APUs (monolithic ones for laptops for instance, not sure on chiplet ones, if we get those they probably won't show up til Computex, but we might see them do something like skip right to Zen 3 CPU chiplet). Possibly Navi 20 announcement with it launching in the spring (or maybe around E3). E3 we get new console announcements. Computex we get Zen 3 Ryzen and then Zen 3 EPYC launches fully end of summer (same time as now). Then the fall we get new console launches. Not sure on Arcturus, its supposed to be out next year, but not sure in what respect. But if Navi 20 were to be out early, they could have Arcturus out for a fall launch. I'm not sure if there'd be any concern about upstaging the consoles, so I could see them possibly waiting for CES 2021 to announce Arcturus. They could possibly put it into production end of 2020 and have it ready for launch. It could also be part of them changing to a new product stack setup (i.e. splitting consumer and pro GPUs), where Arcturus would be a top down consumer launch (and feature probably GDDR6). Or maybe the pro stuff just goes to chiplets with an I/O die on an interposer with HBM).