Third Node or Range Extender?

Recently installed 2 node Velop AX4200 system in our house. Need about 3,000 feet of coverage and has thick walls and 2 floors.

One area of the house gets weak coverage with this setup. Is it better to add a third node or go with one of the Linksys range extenders, which are cheaper? Any cons to the range extender? I can’t tell whether the Linksys Velop extender (WHW0101P) is WIFI6 and whether it will work with AX4200 mesh system. Thanks!