Modem-ethernet to main deco-ethernet to slave deco in basement-ethernet to PC
Yes this would work just fine as you are hoping, also if you only need/want two units the Deco XE75 is a $100 off right now making it 200 usd, but the m4 would be a great choice too, you just get Wifi 6E with the 75s.
This was your solution, if that is not an option than there is no solution.