Have you considered other bridging options besides wireless, like powerline? It probably won't give you nearly the same throughput, but will likely be far more reliable (assuming powerline works at all). Or perhaps MoCA.
Just seems to me that too many ppl try to *force* wireless to solve every problem. And when it comes to wireless bridging, the lack of reliability is sometimes a problem due to how it's typically implemented. You have to be a bit "lucky" to get a truly reliable wireless connection w/ wireless bridging.
FWIW, I use powerline for my smart TVs, and they work great. Granted, they can only deliver about 75Mbps (shared), but that's more than enough for streaming even high-resolution content. And I don't have to congest my wireless network w/ all that data either. Leave wireless for situations where you have no other option (e.g., a smartphone). As I said, you may lose some throughput, but I'd rather do that than have an unreliable connection.
If you insist on the wireless bridge, it might be better to install Merlin on the RT-AC68U and use a watchdog script to continuously ping across the bridge, and reboot if the connection stops working (the fact it registers as connected on the AP side doesn't necessarily mean it's working).
FWIW, I happen to have an ASUS RT-AC68U as well, and I'm currently using FreshTomato configured as a wireless ethernet bridge over 5GHz for the benefit of my desktop, and it's rock solid. So you might want to consider that option as well (at least if you insist on wireless). The only thing that doesn't work is multicast mDNS. But it's not important for my needs anyway. That's another benefit of powerline/MoCA; you're less likely to have multicast issues.
