The code is ready for review and discussion.

Thanks for this!

I'll look over everything soon.

I would prefer this to be an optional dependency, not required. I can do that part though.

A note about the code: it relies on packets sent in order, and MinecraftClient.execute is invoked in order (which are the case for the current Minecraft codebase)

Now OptiGUI is an optional dependency. You may decide to include it in "suggests" or "recommends". And yes, I tested it without OptiGUI and it didn't crash with ClassNotFoundException.
I'm not quite finished with OptiGUI docs, so I implemented the optional dependency stuff, because I know how my mod works.
Additionally, if you want to test it, make sure to download Fabric language Kotlin to run/mods, and extract the jars from OptiGUI jar to the run/mods folder in the project (commons-text and the appropriate optiglue is enough), because loom does something and otherwise you'll get a juicy "Error loading OptiGlue!" exception.

OptiGUI 2.0.0-beta.4 will introduce breaking change. I'll rewrite the code once OptiGUI 2.0.0 full release arrives, and open a new PR. Which will take some time.

After the breaking change in OptiGUI 2.1.0-beta.1, the code has shrunk in size. I have updated the code.

Notes:

• OptiGUI 2.1.0-beta.1 requires Fabric Loader 0.14.19, so I had to edit gradle.properties
• Nested JARs need to be extracted into run/mods
• A full build probably needs to be issued before running the client, otherwise OptiGUI may not load because of some dependency having the wrong version
• Added a code to check the minimum OptiGUI version

To be discussed:

• Add Fabric Language Kotlin as modLocalRuntime into build.gradle so it gets downloaded automatically?
• Log something if OptiGUI is too old, so the user can see it?

Can you port this to 1.20/.1? I'll do another look over then

Not in the next two weeks, I'm having some time off coding and I may refactor OptiGUI again after that. I'll open another PR on the 1.20 branch when ready