You want to have your second router in bridge mode. Something like this https://www.linksys.com/gb/support-article/?articleNum=137888
Will this disable 5GHz wifi and the ethernet ports on my Linksys router, and will the Linksys router's control panel be unreachable once in bridge mode?
You should be good. It's simple to reset the router if needed anyway
It depends on the specifics of the router and configuration, but it doesn't have to. It won't be your "gateway" anymore but it might be accessible on it's own ip. Same with 5ghz no reason it has to be lost
Your network is behind a second NAT. You need to put your router in bridge mode if you want to have the dhcp and firewall disabled.
Honestly the best answer is to get a dump network switch and plug in all of the devices to it. From there you should configure the WiFi router as a dump AP.
Personally what I did was install openwrt on two devices and then setup roaming with 802.11r
Your router looks like its supported if your feeling brave https://openwrt.org/toh/linksys/ea6350_v3
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