You don’t need to spend all that if you don’t want to. If your range is otherwise ok (number of bars/signal strength) you should just need 1. The one I have was only $70 and it’s fantastic and range is doubled vs the company provided WiFi. If you want to be sure you can go with something like this- just make sure whatever you get has 802.11 AX (not just AC)

tplink WiFi ax for less than a hundred bucks

If your house is huge or you want to be extra sure you can get one of the easy MESH systems (which are way better than just extenders) Netgear ax mesh whole-home system for 250

Why are you even leaving the R6700 in place if you are upgrading to a real mesh system? The MK set is WiFi6, something the R6700 will never provide you. I would probably spend a bit more and get the better kit for $225 on sale. Here it is - https://www.amazon.com/NETGEAR-Nighthawk-Advanced-System-Security/dp/B08V3PMGBR

Those mesh systems have a router in them.. 1 router, and 2 satellites... you don't even need the R6700 anymore if you go this route.. The R6700 can be thrown in the trash can or try to sell it to a friend/neighbor for a few bucks..

If you didn't want to spend that kind of money you would be better off spending less money to get something like this nighthawk range extender / mesh point, leaving the R6700 in place, and probably getting the same effect since you are limited to AC1750 standard by the R6700 anyways.. https://www.amazon.com/NETGEAR-Nighthawk-Tri-Band-Extender-Seamless/dp/B07DQ96B95