Maybe it's counterintuitive, but I think the "jack of all trades" role should be given to someone who's very senior, but isn't focused on career advancement.
It sounds like you got stuck learning more trivial things before you had a good foundation in something more substantial (like C or UNIX).
If it weren't for the rampant ageism in the industry, that job might be a perfect fit for someone in their 60s who has already written a few million lines of C and is happy to help less experienced people just get unblocked.
Instead it often falls to the "junior senior" because the "senior seniors" are all pushing 30 and need to work on portable skills.
(Just my observation of the industry, maybe your own case was different.)
I've found a lot more career happiness with focusing on a long-term domain (networking), long-term expertise (testing), and a stable tech stack (python, c, etc). Got pretty close to burnout chasing the latest libraries in the latest languages a few years ago but now I'm very happy making sure the product is cutting edge but the tech stack is stable.
This is what I'm trying to do as well now (move from full stack web dev to something more in the lines of what you're describing). Any tips you can remember on how to make the move?
I spent a bit of time practicing these tech stacks in my own time and fitting them into prior jobs where I could, but honestly it’s worth just getting out there and applying.
As a fellow webpack victim, the new way of bundling things in FE is esbuild and it's better in every way - faster, simpler to configure, easier to understand... and did I mention faster? my god it's so much faster.
Your point about wasting your finite life dealing with webpack still stands, but just want everyone to know that there is a ray of light in frontend.
Compare that to understanding C or UNIX. There are skills that decay much slower than others.