LAN traffic not WAN. So yes your internet is limiting but ONLY to the internet. Otherwise a workstation to a NAS/server could be up to 10x faster

All mine are older but Asus have traditionally been decent. I personally have switched over to using a custom router for none options and capability and a WAP (Wi-Fi Access Point) like Unifi. The WAP tends to be cheaper and generally all one needs to upgrade.

If you're looking for a router specifically a mini pc or open router box can make it easy. Like Protectli Quotom etc. Then run OpnSense on it and will be the last router software you need to learn. Consumer grade stuff is generally mediocre and prices have gotten a bit ridiculous.

If you think you might want multigig LAN ports in the future look for one with the ports or a Thin Client/miniPC/SFF with expansion port and add a NIC to suit your needs.

Should clarify what exactly you mean by not connecting...to the WiFi. Does it connect when plugging in?

Any additional routers should be in AP mode or settings modified to run similar. A router performs specific routing functions so there can be 1 setup as router. You can just run a cable to a switch if that's all you need. Or a switch then add a WAP (Wi-Fi Access Pointl to the switch.

This may help clarify a bit for you, there's a lot there so kick back and learn

Run some 1 1/2" conduit and run some outdoor rated fiber cable from like fs.com etc

This is a conduit overview Outdoor Conduit HowTo https://www.youtube.com/watch?v=6vmX-hona2o&list=PLqkmlrpDHy5M8Kx7zDxsSAWetAcHWtWFl&index=2

This will clarify a bit for you...there's a lot there so kick back Home Networking Basics https://www.youtube.com/watch?v=fjRKID2ucPY&list=PLqkmlrpDHy5M8Kx7zDxsSAWetAcHWtWFl

Yep you need your 2nd router set as AP mode and wired to your Router 1 not the main router.

Or if you had a 2nd run from main router to your 1st. They could have a coupler/pach cord between the 2 at the main router. I always recommend 3 runs to each location.

This will give you an overview of what need and how to set it all up. There's a lot of info so kick back

Yes generally it won't be an issue. But can only have 1 uplink to router or another switch so everything downstream is limited by that port throughput. Generally best to by a single large switch and connect everything to it.

If you have the ONT white box on the wall you which the router plugs in with an RJ45 cord. Then you can use any router. But if the fiber cable plugs directly into the router and it's a combo then you need the exact one. Maybe you can take old in and swap them out

Your best option is use VLANS your consumer router may not do it. So may require setting up an open router and run like OpnSense etc

I believe the highest upload they have is 40Mb, so that's not far off.