Yes, all old devices and IoT using 2.4ghz only.

Damn, I don't find it complex but really kind of mahuan. Seems like getting new (and stronger) router doesnt always equal to better results.
I would do these if I am living alone but not really ideal with family.

are you trying to say to change DNS? does this resolve ping spikes? currently DoH is enabled, using adguard.

It more likely WiFi interefence, change channel for 5Ghz, and put all of yout IoT at 2.4 Ghz.
If got WiFi fairness, enable it as well, else old device which enter power save mode(lock screen/off screen), will bite all the transfer time.

Secondly, you can think about set the old WiFi router by disable DHCP and choose another channel which don't crash with 5Ghz.
For example, WiFi 7 at new router with CH 36 160MHz, WiFi 5/6 at old router with 80 MHz CH 149.
Yes, just connect router 2 via LAN 2.

I don't think you are lacking money to pay 10-20 watt for another WiFi router, right?

If you think too complex, return the router, wait for newer WiFi 7 router.