All three options you list are ipq4018 SoC. Not much difference between them to comment on. The EA6350V3 has more flash (128MB), but 32MB in the other two is enough.
The EA8300 is an ipq4019 SoC - very close in capability to the IPQ4018, but with an extra radio for mesh back haul.
Since you want mesh and have settled on an ipq40xx target, the EA8300 would be my choice.
All four options should handle ~25 Mbps OpenVPN or ~280 Mbps Wireguard VPN since they are all the same ipq40xx hardware.
in reality, I'm not just targeting a SOC, I'm open to all soc ^^
seriously, i don't really have a preference, it just seems like a lot of people use it.
but for me, I will need a good support of openWRt, because I like the system, but level use (notably line of code) I look at the tutorials, and I copy stupidly, so when all is well, it's cool , but when it fails, I don't really know how to solve.
the use will be domestic, but I just came across an article on Wireguard and I find it really good (especially if it's free)
In your case, provided the VPN speed is fast enough for your needs, I would get an EA6350V3 if you don't need mesh, or the EA8300 if you do want mesh some day.
They are both supported well and have relatively recent ARM SoC; they can both be found used for much less than $100 on ebay. I have a couple used EA6350V3 that I picked up for $25 each shipped on ebay. The EA8300 is a little more. There is no reason to spend more for a new router, or for a faster model - unless you need faster VPN today.
Wireguard is great. I use it from the linux desktop and on my android phone. It pulls much less battery power than OpenVPN on my phone. And of course it is much faster than OpenVPN.
It sounds like you are just starting to learn your way around OpenWrt. This is another good reason to stick with a less expensive used EA6350V3 or EA8300. You'll learn by doing and if you make a mistake and brick something, well you didn't spend that much on it to begin with.
FWIW, I started out stupidly copying tutorials myself - still do - first with dd-wrt, then I moved on to OpenWrt to have the same interface and firmware for all our home routers and AP's. I haven't lost a router yet. It's a journey, dare I say, a hobby even? Have fun with it.
oh yes, but for my darling is not a hobby 
i think go EA8300 
you have lucky for the price, on france ( and europe ) the price is 100€
Is there any difference between EA6350V3 and EA6350-EJ?
The 'EJ' suffix sounds like the regional identifier found on label attached to original packaging. eg. 'UK' variant comes with 3 pin UK 230v AC power adapter.
You have to inspect the underside of the actual router to determine whether it is the original EA6350 (v1), v2 or v3 variant.
Another way to perhaps identify a v3 from earlier v2 is from the original packaging. See some photos here:
https://forum.openwrt.org/t/best-openwrt-router-for-50/38795/13
Sorry, but I have no idea what "EJ" designates. As bill888 says, the most reliable way to confirm a "V3" is to look underneath the router for the "V3" after the "EA6350" model identifier. Here's a picture of mine.
Thanks, but it's kind of difficult to investigate the bottom of a router in an online shop. 
If you're very lucky, you might find the h/w revision printed below one of the barcodes - but that doesn't seem to be always there.
I am still deciding between the EA6350 and Archer C7..
On paper, the EA6350 specs is way better in nearly all aspects (CPU, flash, RAM, USB). I don't know about the WiFi performance though (No reason to sospect anything, I just didn't try it).
On paper, QCA9980 in the C7 'should' be better (3-streams, 1300 Mbit/s) than the 2-stream/ 866 MBit/s WLAN of the EA6350v3. However, the EA6350v3 (ipq4018) is part of the Mu-MIMO wave2 generation (QCA9880 is only wave1) and in practice ipq4018 is faster in every regard (the SOC is much faster, the WLAN is also better).
Yeah, I also thought that despite the advertised WiFi speed, the routed speed of the EA6350 will outweight that.
I have both Zyxel NBG6617 and Linksys EA6350v3, I have problems with both randomly freezing here and there and requirng a power/cycle. It happens every few days or so. I have no idea what the issue is or how to debug it. They are running 19.07-SNAPSHOT, r10627-8385d121d0, I will upgrade to something newer soon.
Upgrade to stable with a clean install. I had random freezes with the snapshot as well.
The WRT32X is on amazon renewed for $100, that's how I bought mine, it's been excellent over the past year.
Thanks for sharing about you also having experienced freezes. I have upgraded to latest stable now and am testing...
Unfortunately I have to report I am still getting random freezes on Zyxel NBG6617 on 19.07.2 so I do not recommend it
fwiw, do you use IPv6 because there is a known problem with ethernet controller failing under load on IPQ401x devices?
https://bugs.openwrt.org/index.php?do=details&task_id=2741
I do in fact use IPV6, I will try disabling it, thank you very much for the advice as I was not aware of this bug. Sorry for derailing the original topic.
