Did you ever find out the answer?
Coincidentally, I was trying something similar, the setup works, but if the main router gets power cycled, and the two devices that are set as bridges are not, they do not reconnect.
I have an asus ac66u as main router, default settings.
I have an asus rtn10+ and an rt 12, using default asus firmware, they are set in repeating mode.
They each have one ethernet wired device directly connected by ethernet wire.
The goal is to use these asus devices to make two wired ethernet devices wireless.
I do not want these asus routers to be repeaters, just bridges.
If I power cycle the rtn12 and rtn10+ last they connect to the ac66u and the wired devices are then "on the LAN".
But this a problem, that is, if the main router gets restarted for any reason, everything else needs to be restarted?