If I were you, I wouldn’t be as worried about a first gen AS iMac as you seem to be. Apple’s had years of experience and work on the AS desktop and laptop systems already.
Heck, the 2018+ iPad Pro’s have been better performers than many MacBooks, and they’re only going to get better with additional ram that desktop and laptops will make possible. The only unknown today is how they’re going to work with Windows emulation. I suspect that Microsoft will provide whatever help they can to make it work well, so that they can keep this part of the market.
I’m not even close to being worried that my early 2013 iMac 21.5” will survive until the AS iMac is launched. I do also have a mid-2013 rMBP 15” that my wife has been using for work that needs a new battery, but since she’s not traveling, I’m not going to spend the money to fix it at this point. I’d rather spend that money on a new AS MBP for her next year - assuming they’ll start rolling out in 2021.