You are way too high in idle RPMs. Should be a (normally) knurled spring loaded adjustment on the throttle linkage arm in the engine. You need the PRMs fast enough to keep the engine from dying when in F gear and the boat moving, yet rotation be slow enough allowing time for the gears to mesh.
The reason R is harder than F is that F gearing interface is different since it includes an "overrun" clutch for times when you cut the throttle, on plane for example, and the water is forcing the prop to turn faster than the engine. R doesn't have that problem so the interconnect is vertical sided cogs and slots so you have to have the RPMs slow enough to give the clutch dog, spring loaded cog time to fall into the slot when passing over it.
675 +- 50 RPM is my current engines spec., in F gear, boat moving.