We're very happy to help debug that sort of thing in the Zulip development community: https://zulip.com/development-community/. Unfortunately, basically no modern team chat product provides satisfactory documentation of their data export format (E.g. Rocket.Chat is just a MongoDB database dump). I don't blame them -- all the data of your instance is a pretty complex thing to document, tends to change often, and doesn't get a ton of use to motivate investing hundreds of hours into writing and maintaining good documentation for it.

But it does mean the quality of import tools is limited by how many people have run it and reported all the issues they ran into so that we can fix them. It is not uncommon for our data import tools to need to work around things that are essentially data corruption in the original database.

Our Rocket.Chat import tool was indeed improved greatly in 6.0 last November thanks to a user with a very large and old Rocket.Chat instance submitting a very helpful pull request last Fall, so you may have better luck if you try again now.