Great to read, thank you!
Ar.js can indeed run on a low-end phone, the marker-based AR doesn’t require very intensive computation. Unfortunately, WebARonARCore only works with earlier versions of ARCore — right now, Chrome already supports newer versions of ARCore out of the box, but the API is incompatible, so we will have to wait some time until Three.js and A-Frame catch up with the new APIs.
The only way I found to do marker-less AR on the Web is using some 3D-party product called 8th Wall. As far as I can tell, they compiled their own AR engine using Web Assembly, so it should run on most modern browsers, but I’m not sure how the performance will be on lower-end phones. You can check it out and try for your self, they have a nice online demo that you can run on your phone: