Mixing Mesh wireless access points with non-mesh wireless access points that are sharing SSID

Hello Experts

I have a question about using several Wireless Access Points sharing SSID on the same location

I have a customer that is rapidly growing. They are expanding the area that needs coverage and are getting more people.

They started with a router with WAP (Wireless Access Point). This is an older router that is not Mesh aware. (Draytek 2820n)
They grew in size and added a dedicated WAP. We set it to have the same SSID & Pre Shared Key as the WAP in the router
Both were working well together and the users could roam around seamlessly

The dedicated WAP failed and we replaced by a new Mesh aware WAP (Cambium Networks cnPilot E400)
We also added a 2nd dedicated Mesh aware WAP (Cambium Networks cnPilot 1000)
We set these 2 Cambium WAPs to work in Mesh mode with the same SSID & Pre Shared Key

So they have 3 WAP
* the router's WAP  (non Meshed)
* WAP 1 Meshed with WAP 2
* WAP 2 Meshed with WAP 1
* all with the same SSID & Pre-Shared-Key

Since then the wireless users experienced constant drop outs and various instability. These are random and unpredictable

I am in the process of trying to debug the whole thing by disabling all WAPs except one then repeating the process with the other 2 WAPs

Question 1: do you see anything wrong with the set up as explained?
Question 2: Is there any reason to think that the mixed "Meshed" & "Non Meshed" AP would create this issue?
Question 3: Any other tests I could do?

Any other suggestions?

Looking forward to your responses

Note: I am opened to creating a Gig project if someone has the experience with this equipment