Check also if Viton and/or Teflon, the materials of the seals can be used with R410a.

1000 US$ is too much for such a pump Apoc. For that price, we install much bigger industrial NH3 pumps.

Have you searched Ebay, for example for a hydraulic pump... can whitstand high pressures, seals easily replacable, relative cheap, ... Or a pump out of the hydraulic steering of a car.

Perhaps stupid ideas but who knows???

Instead of a LSF, you can do it optically or with very small heating probes.

It perhaps even can work the way you draw it, without a liquid receiver. You must be sure that there is allways liquid in the LP separator (to avoid cavitation in the pump)