Since you are going to be installing your own software, I don't think backdoors are a big issue.
I wouldn't recommend an i5 though just because it's probably more power consumption and not really needed. If you can get something like a J4xxx series you'll get a good bang for your buck and low power consumption.
I haven't actually bought from aliexpress, nor do I have one of these newer devices, but I do have an x86 built based on a J1900 and I certainly would upgrade to something like a 4 port x86 if I were replacing my current system.
the 6 LAN ports is really not as big a deal as you might think. I wouldn't use the device like a switch but rather I would actually use a managed switch. The managed switch will keep the router from having to use its CPU to forward packets between LAN machines. At a minimum something like the sg108e from TP-Link... going up from there I might look at one of the 16 port TP-Link jetstream switches or a 24 port ZyXel (which I have and it works well).
at a minimum the device should have 2 LAN ports, but most of them you can get 4 ports for not much more money... I'd probably look at the 4 port devices and consider maybe doing a bonded WAN and bonded LAN, mainly for the redundancy because you probably don't have a 2Gbps WAN.
EDIT: looking at aliexpress it seems like the j3160 quad core devices are more or less the best available... the faster 4xxx series seems to be more concentrated in the desktop/media PC market... 3160 is relatively fast compared to my j1900 and certainly enough for all but multi-gigabit WANs, and quad core is better than 2 core... so I might be looking in that sweet spot.