I hope somebody finds this useful. For quite a long time I was exploring the game's EXE (version 1.000) in IDA to understand the game's internal behavior better. Several weeks ago I decided to try making a tool for texture decompression and compression and now here it is:
https://github.com/fgRuslan/Mdk2VqTex

All the instructions on how to build the tool and how to use it are contained in the repo.

This tool lets you convert textures between the game's TEX format and a regular PNG format. I'm not sure that I will be able to write more code for tools that could, for example, work with models, BSPs or something else because it's too hard. Ideally I would want to gather some community of people who are willing to do a significant amount of reverse-engineering to re-create the game's code for modding purposes.
If that ever happens, the work won't have to start from scratch, as I've already made progress in identifying function names, structures, and other related information. I hope that someday this will stop being just my dream :3

By the way, if you don't want to compile the code you can just download the build from the releases page. The latest build for now is: https://github.com/fgRuslan/Mdk2VqTex/releases/tag/1.3