Thank you all for your help! I will add a Julia version of the Birthdays example to the StanJulia/StanPathfinder.jl repo.
One last question (famous last words!). Until the ā\nā issue with num_threads > 1 is fixed, I tried running it in distributed processes and capture the log files separately (which mostly works), but I do get below message:
Warning: Number of PSIS draws is larger than the total number of draws returned by the single Pathfinders. This is likely unintentional and leads to re-sampling from the same draws.