The ability to populate a choice list dynamically from a feature service is not planned of rate next release of Survey123 at this time. We do have this as an enhancement to consider in the future.
Just letting you know that in the latest 3.12 beta builds available on the Survey123 Early Adopter Community we have made significant improvements to large choice lists which improve the survey load time in both Connect and the field app.
On top of this there are now some new ways you can manage your choice lists dynamically and update them without having to republish the survey, meaning choice lists can now be populated and updated dynamically from a CSV file or hosted table shared in ArcGIS or directly from a feature service using the search() function. Please check out the latest announcement on EAC and test out these new beta features with the latest 3.12 beta builds.