Wifi access points generally work in 2 ways. Point to multipoint access or point to point bridge.

Mesh wifi devices do both at the same time. The form a bridge from itself to another access point then also advertise itself as a point to multipoint access device

Mesh access points also imply some form of smarts to prevent network storms and traffic balancing.

Short answer, the answer is no.

The deco’s are a mesh because of how they forward packets to eachother directly. Meshing is to do with how the APs talk to eachother to forward ethernet frame data.

I think you’re confusing the mesh topology of backbone communications between access points with ‘Internal Roaming’, which is how the wireless client devices jump from one ssid to another.

All the decisions of internal roaming are handled by the client, not the AP, and it’s not really that smart.

Not all devices roam exactly the same, but Apple has a clear ruleset they follow how for iOS devices roam. They also details some info about supporting technologies that the APs can provide, 802.11k and v.

802.11r is not required unless you’re doing EAP, 802.1X radius authentication for each client on your network.

So. If you connect the deco’s via ethernet to the same network, they’re technically not a mesh anymore - but they will make for a decent roaming domain. Same goes for your old wifi access points.

They’re only a mesh if they communicate directly.

In the enterprise, we use a radius server to provide authentication and some kind of wireless LAN controller to make smooth transitions between access points. So instead of having to authenticate each time you connect to a new access point (which can take several seconds) you get passed to the next and this happens in milliseconds. Also each access point has an Ethernet connection back to the LAN.

So the problems with consumer Wi-Fi that mesh systems aim to fix. 1. Ease of use. 2. House size and signal loss. 3. Lack of cabling.

Starting with the last point:

Three. Mesh nodes usually speak to each other over WiFi vs having to have Ethernet connections. This is what the word "mesh" means. This also means they can essentially be placed anywhere in a house that best suits the need or design aesthetic.

Two. A single node home router is ok for about a 1500sqft house. 2000 to 3000 you will need 2 or maybe a 3rd node. Also, a 1500sqft ranch style house with a single router on one end will not provide enough signal at the other end of the house. The things we make our houses out of are very good at absorbing WiFi (wood, Sheetrock, masonry) or reflect WiFi (metal ducting, sheathing and even giant TVs).

One. For most people, getting a premade kit, they take home, follow really simple instructions with very few steps and it just works is great. Literally they just work and they do what they advertise.

For your parents, why would you give them openWRT? Better to just let them use the tplink deco as is, they would never know the difference.

You set it up with openwrt although I've never tested it

Mesh networks are definitely a thing. They're common in utilities like water, gas, or electric where you have a whole bunch of nodes (meters) that you want to read and report data back from to a central hub. Those typically don't use Wi-fi though.

Wifi mesh networks exist as well and allow you to stay connected to the same SSID and network while bouncing from access point to access point. They're typically not as fast or reliable as access points that are hard wired into the network. You can't just use two separate AP's and give them the same SSID. You'll run into all sorts of connectivity issues.

I have two Ubiquiti AP's at home that are hard wired in (with POE) and configured to share the same SSID. They communicate with each other over the LAN and devices can switch back and forth seamlessly depending on which one offers a better signal.