... honking gaming-like machine that could be improved with an i9 mother board, but that's about it.
OK, so you have what you need to build fast for Windows and Linux.
Since you want to play with AS, your choices are limited. It is what it is for now. Despite calls that perhaps 8GB is enough, I would go with 16. You don't get to change it later.
The current models are a first effort, and are not high cost machines, and targeted at typical consumers. Good enough. A notebook might make the most sense if you don't have one. The Air is elegant to the point of sexy. It will almost certainly throttle on long builds, where the 13" won't, but you can use an external fan. My builds on even a 2017 iMac Pro ramp up the fan to max after a couple of minutes, so the Air is almost certain to throttle.
Really impressed with the benchmarks I have seen so far, it seems to beat the pants off of Intel architecture, with even the Air outperforming the fastest i9 Macbooks by a bit.
To pay for the upgrade once they come out with models more suitable for software development, I might suggest a modest NVDA/INTC pairs trade (long NVDA/short INTC) on the stonk market with the difference between the Air/13"/Mini you choose, and a decent current Mac Intel model
Build time isn't necessarily all that - especially if you have multiple machines you can be working on something else on another machine during a build.
But from nearly 50 years experience writing software, I can tell you that the two things that IMO contribute most to productivity (or at least happiness) are short build time and screen real-estate. (And, yes, these new models are limited in the real estate department, with more limited multi-screen support than the recent Intel models.)