Are tier 3 modules from the vanilla the max? Wondering if anything similar to the module 6's from factorio extended are in this mod?

No, we don't think to add any new module.

Any reason as to why no new modules? I assume it was because of balance.

The modules destory the game balance, many buildings become energy cheap, others can go over 60 operations per second making the game engine not iterating all recipe cycle correctly. Also, we don't have time to make new icons and sprites.

I was wondering about the same and made a an add-on mod for it. Originally I only created it for myself and some friends but I thought maybe some people might also be interested:

https://mods.factorio.com/mod/Krastorio2_Modules

It makes use of K2s new resources, items and reworked science packs while trying to keep it balanced.

When you reach the point in the game where you have access to the new modules, you will have reached anti-matter and energy wont be a big issue anymore.
The new modules are strong, but also very expensive to craft.
I also "fixed" (more like a work-around) the issue of some recipes reaching the cap of 1 craft per tick.

Sorry about the self-promotion.
A big thank you to You and your team for creating this amazing mod.

We don't like much that modders use the "krastorio" name as a not registered brand useful to sponsor mods, because many users thinks that works are ours when isn't, but all are free to do whatever they want

On the page of my mod I have a paragraph called credits where I explain this:

Credits

This mod is an add-on for Krastorio 2 by Krastor and Linver.
My mod is not an official expansion to Krastorio 2. It requires Krastorio 2 to be installed, but it is not related to the Krastorio Team in any other way.
A big thank you to the Krastorio Team for creating this amazing mod.

Should I change the name of the mod to something like "Additional Modules for Krastorio 2"? Would that be better?

A few days ago you mentioned in this discussion:

Ps: as I said to all, all are free to create mod plugins for K2, to customize the game more, this will be not so hard to do.

and someone created a mod called Krastorio2 Spiderleg so I thought it was fine.

It was not my intention to offend anybody.
I just thought the title should contain the word Krastorio otherwise it will be pretty much impossible to find even if someone would be interested.

edit: Is this called a plug-in or an add-on? I am not really sure?

I changed the mod description to clarify things for those that do not read the long description. It now starts with:
"Unofficial expansion for Krastorio 2 that ... "

I hope that helps.

Don't worry, it's all fine