I’m 29 so a similar age to you OP and I work in a young industry where most have degrees, as do I.
I’m in a position where I hire both graduates and entry level non-grads, and our company policy is we can hire without a degree, but to progress to management (3 promotions from entry level) a 2:1 degree or higher is required and at least 2 years experience in the role. We make this very clear during the hiring process and there’s no exceptions.
However, on 2 occasions in 5 years I’ve gone to the business to make exceptions to the degree rule for 2 of my direct reports to secure them promotions to management despite them not having degrees because they’ve been exemplary in the role and I had the evidence to back it up. I made a clear business case that it was more expensive to lose them than it was to promote them and on both occasions their promotions were granted.
I mean this in the kindest way possible, but reading between the lines in your posts I think you are quite possibly over establishing your value and in the wrong way. No business, no matter how big and rigid will risk losing a well thought of, experienced member of staff over a missing qualification unless it’s an industry requirement. They just won’t.
Do you have a line manager on your side? If so, work with them to put a business case together with very clear working examples of you already doing the role you’re after and your capabilities. The fact you’re asked to train new management staff isn’t enough, knowing the role isn’t the same as doing it. Once you have the business case, take that to senior management, loop in HR and make your case very pragmatically.
DO NOT:
Assume you ‘deserve’ a promotion based on your tenure
At any point mention that you train managers and this upsets you (this comes across bitter)
Bring emotional rationale to the table. This is a pragmatic decision.
Threaten to get another job if they don’t promote you.
If the answer is still no and the feedback you get is still a lack of degree and nothing more constructive, then you leave, take a pay cut but progress your career or you accept you’re over paid for the job you do compared to the industry and enjoy the reduced responsibility for the good money.
Might be harsh, but if you were one of my reports that’s what I’d be working with you on if I thought you were worth it.