@nzben 

To build a mesh you'd need a controller (like OC200). Here's a guide on how to do that at home.

Keep in mind that EAP is part of the Omada solution and that's a business grade equipment.

On the other hand Deco and Easymesh are home network class equipment.

To get more perspective on the difference between these technologies you could check this blog.

  @nzben 

You need a controller, but it doesn't have to be a hardware controller like the OC200/300.  TPlink also provides, for free, software controllers than run on Windows or Linux OS and they also have a Cloud Controller offering now.  Personally, I either run my controllers on a Raspberry PI type platform (similar hardware as the OC200) using the free Linux controller in a Docker Container, or where available, on a Synology NAS, also using a Docker Container.  If you have a device that is running 24/7, you can probably find a way to install the free controller on that.  The controller gets you other benefits like fast/seamless roaming if your AP model supports that.

Just recently, the EAP615-wall joined the list of devices that can establish a mesh (first ever wall unit to be so capable).

Lastly, to be clear, DECO does not interoperate with EAP225's, for seamless operation it's one or the other.

  @nzben 

Understand that 'Mesh' is just the wireless backhaul of an AP's traffic.  The APs will work better if they are both wired.

Where I think you are going on this is that you want seamless roaming, and using the Controller to set up and manage the APs should facilitate this, but the AP's will need to be on the same layer 2 network (ie switchports on the same router sharing the same DHCP).

I'm sure your QNAP supports Docker and you could probably run Linux in a VM on the MIni and do a dedicated Linux install there.  Most of us here use mbentley's Omada image from the Docker repository, he's very quick to keep his container images updated, and they 'just work'.  Further the Docker images make for very simple upgrades.

  @nzben 

Given the parts you will have on hand, I recommend disabling wifi on the Fritzbox, and replacing it's coverage with the incoming 225.  At the same time, updating firmware on the APs and running a soft controller on your QNAP will provide the best possible user experience.  I do recommend fixing your 2.4G channels at 20Mhz and manually selecting the channels as well (ie 1 6 or 11 based on local interferences--if you don't know what I"m talking about, see what channel the controller picks, and decide if that makes sense or not)  If you do run Wifi on the Fritz for say IOT devices, use a different SSID and a different channel so as not to interfere with what's going on in TPlink land.

I'll see if I can find something to help you get the Controller going.  If QNAP UI is anything like the Synology, it's pretty easy:

1. install Docker app on NAS

2. search for and download the mbentley/Omada Controller image, using the latest :tag (it will correctly download the arm64 binaries for you)

3. create a new container from the downloaded image and spin it up, I recommend 'host' networking the first time around, not 'bridge' for newbies

4. after about 15min, the docker image will have created it's located enviroment and MongoDB files and the web page will start responding https://nas.ip:8043

Follow standard Omada setup guides from there, which in your case really amounts to defining your LAN (subnet) and SSID/VLAN mappings then you can adopt your APs when ready.

  @nzben 

Start here with your NAS

https://www_qnap._com/en/how-to/tutorial/article/how-to-use-container-station-3

Once Container Station is alive, go back to my steps above...ie find/download image, create container with 'host' network, start it up

Here's the docker hub link for the image you want:

https://hub.docker.com/r/mbentley/omada-controller#multi-arch-tags

  @nzben 

What is your particular version then?