I don't think it's really about just retraining, it takes years of experience, the right approach, a bit of luck with what roles come around and some innate ability/natural skills that only a few people have. You'd probably be looking at;
Senior partner in a top law firm (so yes, you can retrain if you've already got a degree, you can do a conversion, then get a training contract and claw your way up, but it's years and years plus being actually good at it, well respected, working hard and the right role coming up)
Investment banking (again, you need to be really good at it and do well, our friend was in backoffice investment banking and retired at 35 but he had a PhD in mathematical algorithm type things, you can't really train that, you have to basically be a maths genius - he was picked up after completing his phd at an extrenely well regarded uni and bribed by various banks to work for them)
Cto/senior tech roles - you could retrain/get a degree in computer science or similar and go from there, but you have to be good at it, you then have to work through the ranks till CTO comes up which is going to require leadership skills, strategic vision and technological ability
Senior consultant in a medical field in a private hospital (a parent was a private consultant psychiatrist and made about this, you'd need an MD, then to specialise, then to work through the ranks from being a jnr doctor, then get a consultant role and then be brave and commercially astute enough to set up in private practice and attract enoigh private clients at a high enough rate - so you have to actually be good, get a good reputation and be liked by clients - these are innate skills and attributes plus hard work for years)
Business consultant with a niche skill set (dh is on 350k but runs his own business, he runs lots of contracts with different clients, has an extremely niche sector knowledge and network and as a result charges a high hourly rate or retainer. He does this because he is amazingly good at networking, stakeholder management, communicating complex ideas - you can't train it, sure you can do some kind of degree in strategic business management, but it's commercial understanding, strategic ideas and networking skill that gets you the big dollars)
Ultimately, if you're not top in your field at the moment, and haven't shown commercial, leadership and strategic skills, just "re training" won't really help you earn 250 plus. It's a nice idea that you just do a quick course/a new degree/move industries and bam there you are on 300k, but the reality is that the people who earn that money got there through their unique personal skills, hard work and an ability to get to the top of their fields. Some industries and types of roles are naturally higher earning (information security, investment banking, certain areas of law, IT architecture etc) but for 250k plus you have to be able to lead in those areas, or excel in them, or attract clients in those areas or be brave enough to set up alone and take risks attracting your own clients. Someone who is a mid ranking project type manager can't just do a law conversion course and assume they'll make Senior partner in a few years. Lots of lawyers earn 50-100k (which is not to be sniffed at) and will do that their whole career because they just aren't right for top jobs/areas/client management. Lots of bankers earn good salaries and make good bonuses but just never cut through to becoming exceptionally high earners because they don't have the skills and aptitude to be the best.
Just like everyone in a leadership/management role suxh as yourself cannot be a CEO one day, it's luck, skills, innate ability, hard work and commercial vision that get you there. Not a qualification unfortunately (otherwise, everyone would be signed up to that degree wouldn't they!!!!!)