Hi Deathstar,
I have had a RT-AC68U for 3 years now, I have tried a few firm-wares (currently Merlin installed) - I have always liked the GUI and spec and although my initial impressions were positive subsequently I had a few issues. Whilst I was using it as my main router it would drop the PPoE connections randomly every day, I subsequently found it was a bug that some other users also experienced. I persevered with technical support and different firmwares for about 9 months and then gave up, replacing it with another router (which solved the issue)- I subsequently relegated it to a being a wireless access point.
As a WAP it functions OK, although it has a habit of crashing / freezing and becoming unresponsive requiring a reboot every 2-3 weeks. I have already bought the Ubiquiti WAP that will replace it, just haven't got round to installing it. I already have another Ubiquiti WAP elsewhere in the house and it performs flawlessly.
The RT-68U is an old design, mine is now over 3 years old and perhaps they have made hardware changes since I bought it so maybe things are now better.
My personal advice since you already have a modem would be to buy (or make using an old PC) a pfsense or OPNsense box but I know this isn't for everyone. If you want an off-the-shelf one then I my gut feeling is that a Draytek or Billion would be better than an Asus.
Good luck!
Chunks