Regarding the Desktop/Laptop difference. Yes, you are right to make that distinction and indeed little is known so far about how Alder lake (and possibly DDR5) would get implemented in laptop solutions.
I’m more desktop centric so my timeline would probably be - wait Alder lake + possibly a few months to see what motherboard manufacturers do and aim for a CPU/motherboard/memory upgrade to my desktop some time Q1 or Q2 2022.
However, laptop as an interim solution isn’t such a crazy idea if the price is right and you want to be able to play over “the holidays”
Laptop prices with those specs might droop a little once Alder lake alternatives get launched, but maybe not by that much and that price drop likely won’t hit until the January sales anyway.