leewhat

Is that similar to your internet plan?

The mikrotik probably still in router mode and it conflict when you connect it to existing network. Download Winbox to config it and just plug ethernet from the mikrotik to PC, don't connect to main network first. then follow the part 1 https://tehnoblog.org/mikrotik-router-how-to-convert-hap-or-hap-lite-into-ordinary-switch-or-wireless-access-point-bridge/

jerkiness seems like computer performance issue, or you are used to see high refresh rate monitor.

what speed do you get from speedtest?

if you have multimeter, could check the power adapter.

What is the modem/router model?

There is Flint 2 up for pre-order if you can wait https://www.gl-inet.com/products/gl-mt6000/#pre-order

can follow this https://docs.gl-inet.com/router/en/4/faq/connect_to_a_hotspot_with_captive_portal/

what is the extender model?

disable dnsmasq?

SQM is on router, not switch. like Nanopi R4S and install openwrt.

What you are looking for is WISP mode, not Access Point mode.

just need to keep it simple, PC 1 and PC 2 both plug ethernet cable to the router, done. you will get higher bandwidth and lower latency.