ASCII wrote

I was wondering if the EAP245v3 can be used as Root AP in a mesh network?

AFAIK the EAP245 still does not support meshing. But even if will support meshing in a future firmware version (as has been announced), the answer depends on the region you're in:

In all EU countries, outdoor APs are limited to outdoor channels in the 5 GHz band by law. You need to set this limit in Omada controller if deploying outdoor APs in EU countries. Unfortunately, Omada controller does not allow to limit channels on a per-AP base anymore, but in latest versions it can be set only site-wide. The controller tries to be »smart« in that it just limits the channels for outdoor devices. Since indoor EAPs can automatically select indoor-only channels if channel selection set to »Auto« (or if DFS requires to change the channel even if set to a specific one), there could be an issue with connecting outdoor EAPs to an indoor EAP.

For meshing indoor EAPs together there is no such restriction.

Meshing is currently supported by EAP225 and EAP225-Outdoor only. It has been announced in a future firmware version for EAP245, too.

Note that all EAPs you want to mesh need to support meshing, so you can't mesh EAP225 with EAP245 (yet).

ASCII wrote

On a related note: If all my APs are connected via ethernet cable, is there a good reasoning to keep Mesh support enabled in the controller?

Does Mesh provide any functionality besides connecting APs wirelessly?

If all your EAPs are connected via cable, there is not only no reason to keep Mesh support enabled, but also no good reason to use a mesh to replace the cables.

Meshing will not increase the user capacity of a WLAN, so a mesh network will only support as much users as one EAP alone. Meshing is helpful if you want to increase the WLAN coverage by deploying an additional EAP and a cable connection to this EAP is not possible for whatever reason. But meshing wil also cut the total bandwidth since packets from clients have to be sent twice (or three or even four times depending on the number of mesh hops) over the air.

Reading through https://dongknows.com/tp-link-omada-class-diy-mesh-review/ , it says Mesh support needs to be enabled to allow seamless handoff between APs.

Is that true?

Definitely not! For seamless roaming the EAPs must be connected to the controller, but need not be meshed. The blog author probably got confused by a note in the mesh FAQ which states that extended features don't work if the controller is not running. Mesh and fast roaming both need the controller, but fast roaming also works on non-mesh networks, too.

Note also that meshing still works if the controller shuts down, but re-organizing the mesh on failure of a single EAP will not work until the controller comes back online.

Traditional roaming (not so fast) even works without any fast roaming protocol if the SSIDs, security modes and WiFi modes of different EAPs are the same. Fast roaming just avoids another WiFi survey of the client when moving between basic services sets. The EAP will offer the client a list of nearby EAPs, so it could choose one to connect to much faster.