Thank you both for your answer ! I appreciate a lot all your details.
Concerning the question 2 and 3, I will rephrase.
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I was not sure if the ubiquity mesh solution that is powered supply through the PoE, can also handle a switch. It seems feasible if I'm using the whole product line of Ubiquity, but does OpenWRT can do that on this specific device ? If yes, it'd be great. I'm asking the question because it's not something i'm used to see, and i'm not sure if it follows a standard in terms of software, or if it's a dedicated copyright solution. So if the mesh node is capable to replicate the wireless signal + connect device that are connected on a switch through is PoE, it's awesome!
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If the the question 2 is feasible, then I was curious if the concept of vlan (layer 2) also works on the mesh node for the both parts (i.e., repeated wireless and the switch directly connected on it). It seems okay with UI Controller, but it is on OpenWRT ? If it is not, it's a NOGO for my project.
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I'm pretty happy with the current set-up, I have 400mbps (the limit from my ISP, I was pretty surprise that the final computer had such a performance...). But I would like to control a little bit more the network (vlan part + monitoring). I've seen that Orbi pro (part of Netgear) device seem capable of handling vlan as well as more SSID (4 to be exact right now), but I like the open source world and their solution are very expensive. Also, I've recently seen that OpenWRT is being used on the Orbi devices (https://www.reddit.com/r/openwrt/comments/l0v9gw/does_orbi_netgear_support_openwrt/ and Netgear Orbi RBR50 build?).
In general a wireless mesh network should be your last resort.
Yeah I know 
but due to the way I live (for now and for few years), no holes! So wireless is one of the best choice. I tried power line carrier, but I had bad experiment with old houses.
- I'm not afraid to configure the different parts through cli (iptables and so on), but I am if have to code some part in OpenWRT (network coding is not my field of knowledge, and I would have no time to be part of it).
If you are a first-time user of OpenWRT beware that OpenWRT is not a turn-key solution. Common use-cases can generally be easily configured through the web interface but less common and more complex requirements can require in-depth knowledge of the software. This is different from, for example, a Netgear wireless router where a software integrator has built a Linux based solution with some configuration options for a fixed number of use cases. OpenWRT can do a lot and fulfil requirements that no proprietary vendor can but at the same time it can also require more knowledge and sometimes things might not work as expected.
Thanks to remind me that. I guess it should be okay to spend time to configure. I'm more afraid that the features (vlan on mesh, vlan on switch that is connected on the mesh, wireguard on a specific vlan) do not exist, or not implemented with OpenWRT yet, or that it exists, but only for specific devices (and in consequence I buy the wrong ones).
Therefore, I would recommend to build the new network separately and test it beforehand. Otherwise, the people in your house might become upset if they don't have Internet access and you will blame it on OpenWRT.
I will !