Thank you for including such detailed information about what you're trying to do, that's a rarity here!
The reason you probably haven't gotten many replies is probably that (from what I have seen) most people here are asking for help and not giving it. Unfortunately, I don't know enough about how GUIs work to help you either, but I hope that someone here can!
Keeping that in mind, here's some general advice (take this with a grain of salt):
Split the problem into smaller pieces. For example, start by finding everything you can about modifying the standard inventory. I'm pretty sure that TerraFirmaCraft modifies the inventory, so I'd suggest starting by looking at their GitHub and trying to figure out what parts control their replacement GUI: https://github.com/TerraFirmaCraft/TerraFirmaCraft. Start small; for instance, just change the layout or something to start with. Then, add tabs, and so forth. Once that is done and working, see if you can restrict the inventory from opening, and only allow it to open when the user has a certain item. Proceed slowly. If something isn't needed, delay it for a future step.
For learning Java, here's some links to tutorials and resources (copied from the Forge Discord's bot):
JetBrains Academy (free online course): https://www.jetbrains.com/academy/ Codeacademy (free online course): https://www.codecademy.com/learn/learn-java University of Helsinki (free online course): https://java-programming.mooc.fi/ Basic Java Tutorials: https://docs.oracle.com/javase/tutorial/ Introduction to Programming using Java by David J. Eck (free online textbook): http://math.hws.edu/javanotes/
Those may or may not be helpful, depending on how advanced you are by now.
All the same, best of luck with your inventory overhaul project!