How to hotcoat and sand concaves without all the resin pooling in the bottom of those features?
A 1/8" concave is not all that much and does not need any extra hotcoat work. The resin will pretty much follow the shape of the board and create a shell. A shallow concave like you have will not fill that much with resin sliding down the sides of the concave. Your last walk out or "tip out" with the brush will keep the hotcoat shell pretty uniform in the concaves.
Then it's a matter of sanding. I recommend a Flexpad Soft Density sanding pad to conform to the concave and sand it all uniformly.
Deep concaves (1/4" +) take more sanding finesse as the resin will pool a lot more than a shallow concave.