You are right, of course. I'm still weighing the pros/cons of this approach.
My plan is to use RT-AC5300 as repeater with one 5GHz bands as dedicated backhaul to main router. I believe it is possible to simulate a dedicated backhaul by specifying a different SSID/password just for the repeated backhaul band (to prevent wireless clients from connecting).
By default, repeater mode just repeats the main router SSID on all three bands. But you can manually set different SSID for each band on the repeater. Therefore, for the backhaul 5Ghz band, set a different SSID/password. And use the remaining two bands (5Ghz and 2.4Ghz) to service wireless clients.