If DoH/DoT is configured on the Router, is it also needed to configure DoT/DoH in the Browser or OS?
No, but your OS needs to be configured to use the DNS server on your router. Easiest way to do this is with DHCP + NAT rule to ensure all DNS queries are processed by your router.
No, but check if the browser isn’t using other DoH provider. If so just disable it.
Note that it’s obviously a different story for mobile devices. If you connect to different networks, you might want to leave DoH on on your device
Makes sense, is their any downside if I set the same DNS in Android and Router?
No - in either case a https connection will be established and DNS will be available via that connection. The only exception is the very first connection of the browser/system, where the domain of the DoH provider needs to be resolved first (e.g. dns.quad9.net -> 9.9.9.9).
Depends on your security model IMHO. If unencrypted dns traffic on your network, or your router being a possible aggretion point for dns requests from devices on your network is fine, then it is a great way to simplify using it for your network.
I imagine it’s probably good for 98% of people.