Sewer line slope

I'm about to add a bathroom in an outbuilding that is on a bluff, with the septic tank 40' below, then over a ways. I'm hoping to run a 3" ABS line 40' down the bluff (about a 45 degree angle), then across the flat at standard 1/4"/ft. for another 60' to the tank.

Is there any problem with the water bypassing the solids on this angle?

No, the water leaving behind solids is a Myth that has been busted!
1/4" per foot is a minimum pitch.

Nice handle by the way!