Mesh is very different.
Roaming networks are simply multiple access points (extremes in your case) connected to an Ethernet network, providing multiple bridges to the network. Data sent and received by clients connected to the remote access point is bridged from the wireless port to the Ethernet port then sent via Ethernet to the router (unless the data is being sent to another client connected to the same access point, either wirelessly or ethernet). Ethernet runs of less than 300 feet should experience virtually no slowdown between access points.
A Mesh network uses WiFi between access points. A mesh device will have a radio dedicated for links between access points, and a second for client connections. The standards for Mesh WiFi are kind of loose and each solution seems to approach things a bit different. But, one thing in common is the two radios.
In WiFi networks, less is almost always better. WiFi is particularly troubled by attenuation (distance and obstacles, interference (from other radio signals in the same frequency ranges), and contention (simultaneous packet collisions at the access point). More radios can increase both interference and contention, though Mesh networks generally self tune to avoid overlaps as much as possible.
To fix the dead spot, consider placement of the Extremes. Your home is likely long and relatively narrow if it is that large on a single level. While placing them far apart helps reduce overlap\interference\contention, it might be leaving a weak signal in the middle of the home. Try moving them closer by a bit so each is centered on respective ends of the home.
Wall construction also impacts signals. Thin wood frame\sheetrock allows signals to pass better than thick, mason or steel frame walls. This would impact Mesh networks even more than Roaming, as the WiFi links between access points would be slowed by the obstacles.
If construction materials or impractical relocation of the access points is troublesome, consider a third Ethernet connected access point such as an AP Express. While the Express doesn't have the faster 802.11ac (it is limited to 802.11n), it could be just the thing to fill the gap in the weak signal location.
If your ISP service is greater than 300 Mbps (not likely unless you have a fibre 1Gbps service), a third Extreme might be desirable if speed is critical in the current weak signal area. Extremes with 802.11ac can achieve up to 1.3Gbps, but 802.11n (express) will max out around 300Mbps. But, if your ISP is 100Mbps for example, all the speed in the world on your WiFi wont speed up internet speeds as the ISP link would be the bottleneck. However, if the greatest need is large file transfers between wireless devices, go as fast as you can with all access points.
Don't over invest. 802.11ax is due to hit the market in the next 1-2 years and will be even faster and less troubled by contention, and with longer range. Not sure how Apple will deal with that given their network engineers have been re-assigned to other products and are not currently actively working on Airport gear. That may change, but for now, it is an uncertainty. Regardless, when the ax standard arrives, new gear will be required to take advantage of it.
