Hi there Mfor3
I always think there's a big difference between being organised, efficient, business-plan-driven, cut-throat in business or work (and rather fancy myself as all of these things ... although I'm probably really not so cut-throat as I would want!) - and in personal life where having a 5 year plan and all that sort of thing is perhaps not really so applicable (for example, I remember once having lunch with a girl who said 'my aim is to be engaged this time next year and married the next' - hehe!).
The thing with your dilemma is that it brings this conceptual split between private life and working life into focus, as with all questions re work/life balance, doesn't it?
But in the end (I think, anyway) it comes down to gut feeling and what will make any of us feel happy.
I haven't had a baby yet (another 6 weeks to go) so whilst I'm not qualified to offer an opinion on the experience of dealing with conflicting feelings about family and work, I do have an opinion about that old saying you mention 'no-one ever wished, on their death-bed, they'd spent more time at work'.
My own particular tangent on that is 'no-one ever wished they'd spent more time doing paid work they didn't really like'.
Lots of people have jobs they don't really like but aren't in a position - location, skills, experience, available opportunities etc - to change them. But for some lucky people, like the ones who post to this topic, there are opportunities to change ways of working/types of job etc.
So maybe the more big question for you (or anyone reading with the same question in mind) is not about whether to stay in the GP partnership or resign and continue working in the practice as a locum etc but whether you want a much bigger change - no paid work at all for a bit (if you can afford it), or having a go at doing some slightly different work in the same field which fits better with your family requirements and won't mean a status change/demotion with regards to the partnership you're already in. Hmmm.
So my answer to the questions you originally posted stays the same - don't resign etc. But if you want a life change, well that's a completely different matter. And as you must be a highly qualified person with sought-after skills etc you're in a very good position to achieve the changes you want
(Gosh, reading this back to myself I sound like some kind of low-rent Oprah! )