Note to reader: Before you comment, generic books like "Learn Linux" do not meet my requirements.
I work as a jr. QA engineer with a custom Linux OS, I've picked up a lot of stuff on how to check for segfaults, write effective test cases, yadda yadda.
I feel I'm selling myself short by not having strong knowledge of Linux at the low level, I'm not interested in courses that explain "This is how to set up X,Y,Z, or this is 'chmod' it does this", I'm after something with a bit of meat on it to provide improvement to my bug-hunting and writing of my test cases based on knowledge of low-level Linux.
I started learning C after writing god knows how many Python/Bash/C# programs and feeling like I need to go to where it all matters, however, ask anyone, some C is tough as **bleep**, and I don't fancy myself as a C software engineer
I've expressed to my line manager before that there's a field out there that I know is rich with knowledge but we aren't given training beyond what is essential. I was told "you are not a kernel developer, so this information is pointless".
You might check out LFS.
"Linux From Scratch (LFS) is a project that provides you with step-by-step instructions for building your own custom Linux system, entirely from source code."
Also, https://kernel.org has lots of documentation.
did you check the performance tuning, and troubleshooting courses, as well as the container secuirty? they might be of interest to you? if you are more interested in kernel development, Linux foundation used to run courses related to development of linux, kernel modules. check them out
A collaborative learning environment, enabling open source skill development.