I dont know that you specified this anywhere, are you looking for a DDR4 board to reuse your memory, or DDR5 so you can get newer memory? If its DDR4, those are now slightly harder to find on the new market
This is an important distinction. Both DDR4 and DDR5 motherboards exist.
The benefit of z690 boards is that they are discounted but still offer more or less the same stuff z790 boards do. High end z motherboards can be had for entry level money if you're shopping a generation back.
The AsRock Extreme sold for $120 before stocks ran out. Of the remaining motherboards, you can have your pick. They're almost all cheap.
And they will happily work with a 14th gen Intel after a BIOS update.
Intel 12th gen CPUs are also heavily discounted so it makes perfect sense to use one of those CPUs. 13/14 gen are a cut above and perform better. But z690 and z790 motherboards are basically the same with z790s being mostly DDR5 and with maybe an extra NVMe slot.
It is very wise to look for ATX only motherboards, but the question of "stability" is kinda off. All motherboards are stable as long as they're not dirt cheap, bargain motherboards. That's limited to H610 and some super cheap B660 and B760 chipset motherboards. z chipset boards are flagship chipset boards. Nothing to fear there.
If you want to install a i9 CPU and overclock it (not worth your trouble when it's performance is maxxed out alread; you get 3-4% at most) then it would be better to avoid anything with less than a 12 phase VRM, but phases aren't everything. Quality and spec matters too.
In my opinion a 60+A, 8+8 setup is enough though there is no reason to say no to more, like 100A, 10+10. For an i5 CPU, and a locked one at that, a 6+6 would be fine, even overkill maybe. 6+1+1 though
That would be something to avoid.
I would MUCH rather have a backup BIOS and/or CPU-less BIOS flash capability, or fast USB-C ports or headers, than look too closely at VRM. It's an important part of the motherboard specs, but examine all the details.