I would not recommend using RCA-to-XLR adapters unless you absolutely had to.
The idea of using the switch on the back of the A51 to switch between RCA and XLR inputs is likely your best result. It's really not a bad idea and you can keep all cables connected at the same time (both RCA and XLR). Unfortunately, the switch on back of A51 is global and you cannot configure each input separately. Otherwise, just use only the RCA outputs on the Oppo and live with that.
Another idea is to sell your A51 and then look at buying A31 and a A21 amps. The A21 can be setup to use XLR inputs for your left/right. The A31 would use RCA for all remaining channels.
Or just buy an A21 right now for left/right.
The idea of using the switch on the back of the A51 to switch between RCA and XLR inputs is likely your best result. It's really not a bad idea and you can keep all cables connected at the same time (both RCA and XLR). Unfortunately, the switch on back of A51 is global and you cannot configure each input separately. Otherwise, just use only the RCA outputs on the Oppo and live with that.
Another idea is to sell your A51 and then look at buying A31 and a A21 amps. The A21 can be setup to use XLR inputs for your left/right. The A31 would use RCA for all remaining channels.
Or just buy an A21 right now for left/right.