What matters here is not the numbers of routers, but the experience in multiple countries handling such setup. In another country I have PPPoE fiber connection, it was initially running on RT-AC68U but it has sometimes dropped PPPoE session without any reason. It was switching to 2nd WAN but never restored back to WAN1 when it was up and running. I am not sure which cases have you seen but seeing and experiencing something is totally different. What looks great on paper, does not always have the same appearance in real life. I replaced it with Synology RT2600AC, which is absolutely the best home router for dual wan setup. Works rock solid for many months already, sending me email notifications about the current WAN state. They had one bug related to DNS which I submitted to them, it was causing DNS to fail due to dual wan watchdog. They acknowledged the bug and fixed it. If I would not require Astrill applet (which btw doesn't come from my inability to setup ovpn manually), I would be only using these routers. Synology is amazing in keeping up with firmware releases. Their SRM firmware is excellent. I am sorry to say but Asus is in a different league than Synology.
There is nothing specific about my ISP, since no specific setup is required from standard PPPoE implementation in Netgear stock FW and OpenWRT. It just works. So if it works well on 2 different routers, running 2 different firmwares, while ASUS (after NVRAM + latest Merlin loaded from the recovery gui after WPS reset) requires tweaking some options to make it work properly, then just think again about what you wrote. It is easy to write things when they are working, but if you would be seated here, having mine problems with this simple setup, then I am not sure if you would be so happy about ASUS.
log a ticket with Asus and send feedback to them