"There is a lot of talk on here about how absolutely awful new parenthood is. Why do you bloomin well bother then ?"
Because parenthood is about more than the first 3 or 4 years, obviously. (Also, it's really hard to put them back once you have them.)
You missed out on those very early years with your DSD, so perhaps you don't realise how brutal they can be on a relationship. If the new relationship you're contemplating embarking on is anything less than rock solid, it may quickly get into serious trouble.
This is something you should think hard about, as someone who is considering trading a relationship that was (presumably) founded on love and mutual desire to spend your lives together, for one where a driving factor is that your partner can produce children and is keen to do so quickly.
The problem with picking a partner based mostly on their firmness of skin, distance from menopause and desire for children, is that those attributes can cloud the fact that at a deep level, you may not be intellectually or emotionally suited as a couple. You think you've fallen in love, but really you've found someone who ticks some time-sensitive criteria and your subscious will very quickly translate this into something more rationally palatable.
This can be a problem for you, but possibly more importantly, it can be a problem for your younger partner. Women tend to become stronger-minded as they age. After you emerge from the haze of the early years (which may take half a decade or more depending on how many children you have), you may both find yourself staring at the other person and wondering who the hell they are and what you have in common. There will be a lot of work to do to keep that relationship alive.
Since you find yourself at a point where the love you felt on the day you married your wife is not enough to push you through the challenges of confronting the natural changes of aging and (possible) regret for missed chances, then I ask you to ask yourself if a mutual desire for children is likely to be enough to make a second partnership last with someone who will, frankly, have more options available to her than you will.