First, great job over the many years!
Sounds like you need a classic Product Manager. Someone that makes the decision on what features to add (or remove) in future releases and, more important, when models go End of Life.
I am amazed at how many routers Asus continues to support. My hats off to both Asus and yourself. Of course Asus has a development group (likely backed by PM's) and you are a single dev.
My recommendation - begin to EOL older and less popular models. Simplify.
Folks, if you are running a 5+ year older router and continue to expect enhancements - time to move up and on.
Also, give some due and financial consideration to Merlin. Go to this thread and act:
http://www.snbforums.com/threads/the-rmerlin-donation-thread.17285/