If you want to use something similar to AR Codes on the 3DS you have the following options:
NTR: This CFW actually uses the cheat.plg files you already mentioned. Those are, in my opinion, the closest you can get to AR codes, the plugins run alongside the game so codes that permanently write to the RAM are possible (e. g. Infinite Health codes). It supports pointers and pretty much everything there is. The only problem is that there's no big respositories to get those plugins like the usrcheat.dat database we had in the DS era, the only website that shares many cheat plugins is completely in Chinese and most of the plugins there only support the Japanese version of the games. For me personally, that's not a huge issue since I'm familiar with programming and I like fiddling around with things like this so I actually enjoy making my own plugins, but for people without programming skills and an affinity for such efforts these could be pretty difficult to make by themselves.
ARCode: This was already patched, like you said. I never had much use for it anyways, since this doesn't run alongside the games so the codes have to be executed manually everytime you want to use them, which means that a code which gives you infinite health would not be possible, you could only make a code which fills your health, that you'd have to run the browser exploit for everytime you'd want to use it
Gateway: Contrary to what you said, it actually does support the New 3DS. The advantage of this system is that it's the only one to date which supports finding cheat codes right on the 3DS itself, similar to what you could do with a DSTWO and the DS. However, it has many disadvantages like the already mentioned fact that it doesn't support pointers. It also still has many bugs in the cheat engine which often result into your cheat list not saving correctly etc.
So all in all, I'd recommend cheating with NTR CFW.
