I was looking for some mods to allow me to make my own mercenary guild; however, I don't seem to find one capable of doing so yet. I do know that Sokco's Guild Starter has been ported to SSE, but it's missing options to allow to create a proper mercenary guild in my honest opinion. Also, the original author has not worked on the mod for years and has been abandoned, so I assume that we can't do any edits without his permission.
There is another mod in oldrim called Guild Master created by yakusai. Although, he also abandoned the mod due to irl issues that he had, he gave us permission to make edits to his mod. It doesn't have any way to create a mercenary guild there yet though.