So. I've found the code on Github, and found references that say I need to run Dolphin 13.x - So, installed 13.1 now, and got the app up. Found that there were errors in the migration tool if running later than php 8.1 due to deprecated functions, so now running on 8.1

However, my dolphin site has a large number of users, 65188 in total. The spinning logo eventually stops, and if I reload the page, it shows status or Profiles as "Already started",but nothing proceeding or finishing. I left it overnight, and still the same status.

Any hints?