There's absolutely zero point in buying a Wifi 7/BE router unless you're specifically buying a tri-band one. If it doesn't support 6GHz out of the box, don't buy it. I think the cheapest tri-band option on the market rn is one of Ubiquiti's UniFi APs.
Others have explained that mesh mode is basically just disabling DHCP on the downstream router and using the same SSID and password.
Truthfully, the most important thing you can do is wire everything stationary to Ethernet.
The only objects on my wifi network are phones, laptops, a tablet, and some ESP8266s and ESP32s doing smart home automation stuff through Home Assistant.