Digital is supposed to be digital.
So if the signal going through the HDMI cable from disc to TV is completely digital, I can't see why you need to use the second HDMI to bypass the receiver unless:
1. Your receiver won't allow 4k pass through at all,
2. Your receiver won't allow pass through of the latest and greatest HDR codecs,
3. Your receiver in some way affects the picture in a way you don't like (in extremis, say all the grass looks pink or something or the motion is juddery).
The original intent of putting dual JDMI outlets on both the new UHD BDPs and long long ago on regular BDPs was to get around problems with older generation receivers that couln't handle newer video codecs, frame rates or resolutions. Nothing more.
As for the analogue audio outputs on the Oppo, it's really up to the OP to try both and see which he likes best, provided, of course, his receiver has the requisite analogue inputs.
I do wonder, however, how Atmos is supposed to work, given the 7.1 output is probably 2 dimensional and doesn't include the overhead channels.