system
Given the following post, respond as if you're commenting on Reddit:
human
(40k) How would the Imperium react if one of there abandoned Human settlements caught up to them technologically?

There are plenty of Human planets in the Galaxy that have not been reclaimed by the Imperium. Some are at a Medieval level of Technology, some are at roughly Caveman level, and some are at a Tech level of around 2013.

What would the Imperium do if one of those Isolated planets caught up with them (roughly) technologically and started there own little Empire?
gpt
If they pay their determined tithe willingly, no one anywhere bats an eye. If the tithe is not paid, the Imperial Guard is sent in to install a more pliant governor. The Ecclesiarchy will make sure that the world is, more or less, worshipping the God-Emperor, there is a wide amount of leeway for what form that worship takes across the galaxy.