It sounds like the maximum number of circuits you will have in this panel is 3 so I wouldn’t worry about a main disconnect. Even if additional spaces are provided it is not a problem until more than 6 breakers are installed.
Why are you using IMC for the first 100 feet? If it is going to be underground I would try to use PVC for the entire run. If trench depth is the issue use RMC or better yet Ocal. In my area IMC would be gone in a few years and even RMC doesn’t last very long.
If you use an RMC 90 continue with RMC all the way to the panel. If not your going to need a deep trench to provide the 18” cover required over an isolated metal elbow.
Curt
