Saint-Rimbert wrote

We have an external outdoor CPE Pharos (TP-Link TL-WA7210N), with an omnidirectionnel antenna (TP-Link TL-ANT2415D)

TL-WA7210 is not a Pharos CPE, it uses another (older) firmware.

My questions are simple :
- Do we have the right approach to provide internet to the 4 houses ?
- Do we have the proper material to insure a good level of service ? If the answer is no, what should we use ?
- and finally what are the correct settings we should enter to optimize the level of service ?

No. Problem is not the TL-WA7210, but rather your client devices, which hardly can connect over a distance of 60m due to their weak antennas. As you did experience, 30m in direct line of sight works with most client devices, but not if the device is inside the house (too much attenuation). What's more, TL-WA7210 is an old device, end of life reached already.

A better topology would be to use directional links between the houses and the main AP and inside the houses an indoor AP to distribute the WiFi signal locally.

If it's possible to use a cable from the ADSL box to the main AP, I would place the main AP in Villa A and set up links to Villa B, C and D. To keep interference low, I would use 5 GHz devices such as the CPE510 for the direct links and cheap 2.4 GHz indoor APs in the houses wired to the CPE510 client. Problem with this solution is that you need a pair of two CPEs for the link between Villa A and D (one AP, one client) and at least three CPEs for two links from Villa A to B and from A to C (one shared AP, two client devices). Reason for this is that CPE510 has only a built-in directional antenna with 45º beam width.

Another solution which could work is a WBS510 with sector antenna TL-ANT5819MS (120º beam width) at Villa A, which probably covers the area for the three houses B, C and D (still equipped each with a CPE510). But since costs for a WBS and the antenna are somewhat higher than that for two CPEs, the first solution would be preferable, especially because the link between A and D could use full bandwidth and the link between a second device at A and devices at B and C could share the bandwidth of this link.

Anyway, for local distribution you should use an indoor AP, could be even a cheap device such as TL-WA901ND.

Saint-Rimbert wrote

- Because of the topology and vegetation, I think I should install 3 CPE at the central point (red dot), and 1 CPE client in each 3 houses (That requires 6 CPE, but there is a clear view to all 3 houses from this point). Is there any issue (interferences) if the 3 CPE are installed on the same mast, facing different directions ?

In my experience there are interferences (even if set to different channels) if pointing in the same direction. I think, in different directions interferences are no big problem. Vegetation is more critical for 5 GHz devices than for 2.4 GHz devices, so ensure that there is free line of sight (i.e. no trees in between).

Probably you can reduce the amount of APs at the central point to two CPEs if they two houses fall in the 45º beam width of the antenna. On the other hand, three CPEs give more bandwidth, so the decision depends on the uplink's total bandwidth. Usually, a link between two CPE510 can achieve up to 80 Mbps data speed (full duplex, that requires 210 to 240 Mbps half duplex wireless speed).

- connexion between CPE510 and indoor AP : I understand it is possible to use a wired connection, and this will provide the best quality. Is that exact ?

Right, cable is always best quality. BTW, make sure to ground the CPEs as outlined in the installation guide. Ubiquiti offers an CAT.5e outdoor cable called Tough Cable (Level 1 is better with CPEs than Level 2 variant), which has an additional ground wire. If your antenna pole/mast is grounded already, you don't need the ground wire, but an UV-resistant cable.

- SSID : is it possible to use the same SSID for all CPE and APs ? or may that option cause problems in areas covered by 2 APs or CPE ?

Won't be a problem for the CPEs, b/c they not only use the ESSID, but can additionally be locked to the BSSID. But I would recommend to not use same ESSIDs for the direct link and the indoor APs (if you use 5 GHz indoor at all), since client devices could try to connect to the direct links instead of to the indoor APs. If this is what you desire, then you could use same ESSIDs for all APs, indoor and outdoor.