currently using TP-Link AX20, if i want to extend the range, should i buy a better newer router or just use range extender RE705X and activate OneMesh?

double storey house, 1400sqft only.

buy the AX72 or 73  depends on  the devices u have ,  no need of mesh .
my DS  is about 1700 sqft . with unifi 100 M
my router is place in center of house  on glass cabinet top /ground floor
i get 100Mbps/speedtest on AC  in all 3 bedrooms,
i get -60dbm signal  in all rooms / use wifi Analyser app on mobile. 
Correct Antenna placement helps get better signal.

https://www.blacktubi.com/review/tp-link-archer-ax73/

for your budget, asus tuf 4200 wifi6 is still relevant.

3.3 sales from lazada and shopee official store