I moved from teaching to accountancy after 5 years of teaching, so a bit younger than you are and before I had children. I did a big 4 audit trainee role with ACA qualification and thought that the training was excellent.
Unlike most training in industry (AFAIK) you get a lot day release to attend college, so that makes life a bit easier. There was pressure not to fail exams and some people did have their contracts terminated for failing exams. I found my teaching background useful for the exams as I really listened to the tutors
. I think I would struggle with the amount of exam preparation work now that I have children. But I don't think that would be better for any of the qualifications.
I worked at a regional office, rather than London, and got the impression that we had more smaller clients, so got to do a bigger range of work and better understanding of their business than you might on larger clients, thought I did work on some larger clients too. The larger ones often had tighter deadlines and the hours were long at times, but not constantly.
I don't think that applying as a slightly older candidate would be a problem, as long as you wouldn't have an issue with most people managing you being younger than you. But if you are in management in teaching I'm sure that some of the teachers you manage are older than you, so you have experience from the other side. There is a push for diversity in many organisations and that can include age.
A downside to training in the Big 4 in audit can be the amount of travel - you may find you have a long commute to client sites or that you have to work overseas. However I found that you can negotiate and they will take family commitments into account. I moved into industry after qualifying as I wanted to reduce the time I spend travelling. if you like the sound of the Big 4 but want less travel then the tax teams are out less. But if you want to leave practice, fewer jobs are available for those who specialise in tax.
Which qualification you take depends on what type of accountant you want to be (to a certain extent). If you do CIMA then you are likely to work as a management accountant in industry. ACA qualified people seem to work in all sorts of roles both in practice and industry. I think that our senior leaders are mostly ACA qualified. The Big 4 name on your CV does open doors at times.
The amount of maths involved in accountancy is fairly limited - it is mostly arithmetic and percentages. The only bit that I can remember being a bit harder is discounting. But that doesn't come up until later exams and the college tutors know that it is the more challenging maths In the course.
The money is good, though I am currently on 'the mummy track' a bit because I don't want to work FT or travel far.