Extend the wifi signal while using an Apple Airport Extreme Base Station and Frontier digital Fios router

I have 2 Apple Airport Extreme Base Stations. I also have added 4 airport expresses, hoping to extend my wifi coverage.I have ethernet cable that runs directly to the 2 Airport Extremes from the Fios router. That ethernet cable also runs directly to my Mac Pro. I have consistently great signal coming directly off of the ethernet cable but the wifi coverage seems to be worse when I add in the airport expresses. The speed coming in via the ethernet cables and directly off of the router is always around 500 Mbps, both upload and download (that is the service I pay for). The airport expresses run at about 15 - 20Mbps both up and down - terrible!!

I do have one more open space on my Fios router with an ethernet cable that I can run to one of my airport expresses. Is there a way to extend the wifi signal directly from the router? Can I connect an ethernet cable to a third-party wifi extender that will not interfere with my Apple Base Stations? I have tried the Eero and do not like the fact that I cannot connect 2 gateways. And the mesh was not happy working even 30 feet away. I do not have a big house but it is long, and old, and built with those thick lath and plaster walls and the wifi signal tends to get lost even though it might only be 50 feet away.

I know that the Apple signal is proprietary and I have turned off the wifi on the Fios router but that wifi does exist. And it does work. I just wasn't sure if it was interfering with the wifi from the Apple wifi.

And one last question, does having more airport expresses actually slow the network down? It seems to be the case - I just added 2 more and definitely noticed a small drop in speed.

Any help would be greatly appreciated - thanks!