In my view there are all sorts of variables. Some subjects require more or less preparation than others; some subjects' syllabuses change more frequently than others; some departments have different expectations; infant, primary and secondary school teaching have different and variable workloads throughout the year; different schools have different kinds of work in them (a high proportion of students with SENs will obviously require different kinds of planning/preparation than a high proportion of students bound for Oxbridge, and some schools require lots of afterschool/weekend participation and some don't). And yes, some teachers aim to improve and develop their teaching every year and work very hard at doing that - some might be reinventing the wheel and might not be better teachers because of it, while some might be just the kind of hardworking, proactive, dynamic and responsive teachers everybody wants for their children.
As other posters have said, there are clear pros and cons to teaching: long holidays aren't always a bonus since many teachers work over their holidays (DH breaks up tomorrow but that comes with a ton of coursework marking for years 11, 12 and 13 which will easily take him a week of full time work; during half terms he is usually on school trips abroad which are exhausting and demanding, and during the summer holidays he is always asked to sort, re-make, and upgrade departmental resources and to produce all the necessary data post exam-results, etc.). But still, they are much better holidays than most workers get and he thoroughly appreciates that. Weekend working is also at a minimum. At the same time, work during term time is inflexible to the point where he had to apply in writing for two lessons off to attend an ultrasound and then justify himself to the headteacher, and he brings home hours' worth of work every night. Short term deadlines are stressful in any profession, and teaching is one of those professions that includes continual short term deadlines.
I don't think teaching is incompatible with family life (am only pregnant with DC1 but we know plenty of other teachers with happy family lives). But I think the OP is right to think that, while the general perception is that teaching is a family-friendly career option, it is often more complicated than that.
I am not a teacher, but I am an academic. I have great flexibility, brilliant childcare options, and am massively grateful to be in the career I'm in. I also work 70-80 hour weeks and am terrified that my flexibility vs. DH's relative inflexibility won't be a great mix (i.e., it will always be me who has to take time off/juggle things around because I can, while he can't - and always me chasing my tail to catch up). So it all depends on the mix, I think.