Great that Pockethold worked with your host, alas, not great that you still get the 500 error.
As @luceos mentioned, composer takes into consideration your servers environment when download all requirements. Pockethold uses composer, so, in theory, as long as pockethold works, you should not need to download it again locally. (Although, even though I make pockethold, I recommend strongly to find a host with SSH access if you plan to run flarum in a larger production)
Do you have access to an error log of some kind? What PHP version is being used? What controlpanel does your host use?
Edit: as jordanjay29 said earlier, flarum don't support it, but I will as far as it is possible. You can reach me on Discord if you want as well.