I was trained by RAPC as a batch mainframe 3GL programmer. Nobody ever confirmed I actually passed the aptitude test, but I took the posting and got a Civvy career to move straight into. It kept me gainfully employed for 30 years before someone realised I wasn't the world's greatest software engineer, and I took redundancy voluntary separation as early retirement from one of the world's biggest companies last year.
Why, then, you may ask, can I offer any relevant advice? Because we got industrial trainees every year. Karen (must be in her 30s now, got an offer to come back, she had twins and i believe she left to look after them) told me this.
Universities haven't got a clue. They teach you Java first, cos it's easy, then then teach you C++. It's all arse about face. Java is so easy and does so much for you, that it's easy to be lazy. Then, when they teach you C++, which is much harder and you have to code everything yourself, you discover you have to unlearn bad habits. A good C++ programmer could walk Java code, but not vice versa.
I pass on this information for your use as you see fit.
Personally, I'd say learn PLX, JES2 JCL and C++ and get a niche job at IBM, but they won't thank you for it. They can't won't spend enough getting universities teaching mainframe.
A few years ago I read that whereas in the USA, some 138 universities taught mainframe skills, that figure in the UK was, well, zero. Even in America they don't have the skills to replace retirees that have kept the mainframe infrastructure running for decades.
Instead, they dumb down all the good stuff you get from a mainframe so that any jack PC programmer can sort of make it work without the mainframe benefits.
30 years on, I think the prediction that the mainframe is doomed has finally been allowed o be set fulfilling.
Miss it? Not a bit.