RT-AX59U bit limited on wi-fi range any recommendations to extend PROPERLY via ethernet not an issue.
Depends on how fast your connection is and how demanding your needs are.
I've been using a pair of Unifi Access Points which are good up to about 400Mbps. They're a bit of overkill for a domestic environment, though.
For my dad's place (with a 50Mbps connection) we're using cheap travel routers in Access Point mode (GL.iNet Opal (https://www.gl-inet.com/products/gl-sft1200/)), all interconnected via 1000BaseT Ethernet. If you catch the sales you can occasionally find them for less than £40 on Amazon.