Whilst I like my job, I’m curious about the platform and I’m wondering if it could be an avenue for sustainable, secure and well paid employment (I could be wrong).
Since you're an enterprise consultant already, you probably already know that there's always an opportunity to cut down expenses/technical debt, while providing justification for past decisions made by the company (after all, they've probably invested quite a lot in that infrastructure, so these decisions couldn't have been wrong...)
Depending on how deep down the rabbit hole you dare to go, there may be a pot of gold waiting for you or a black hole that sucks the livelihood out of your body. After all, it's mostly a legacy system. Don't expect entirely new systems being written for those niche operating systems. But maybe you find your very own niche in there.
I know a company that I worked with in the past has their main business powered by IBM mainframes, and they may well keep that system going for the foreseeable future. A lot of code may already be running on zSystems Linux or be converted to run on it. At that point, you largely only have a different processor architecture to deal with, and some minor, yet annoying incompatibilities and restrictions that you will encounter soon enough.
A good starting point to learn about IBM z may be "IBM Z Xplore" (https://ibmzxplore.influitive.com/), which gives, apart from an online tutorial with challenges, some free access to IBM z systems. Also check out IBM ZD&T for Learners Edition (https://ibm.github.io/zdt-learners-edition-about/).
Don't expect that IBM responds to any of your inquiries unless you already are a paying customer.
I Googled "IBM zOS certificiation" because I figured that IBM offers training and because getting certification is antithetical to HN's zeitgeist.
I got this, https://www.ibm.com/training/certification/C0005400 so though this might not be what you want, it is a breadcrumb.
Good luck.