Agreeing very much with all this, while also noting some women are more comfortable with the prospect of surgery for cosmetic reasons.
OP, you say I am around 5 foot 8, a size 16. Me, too - and while I'm not overweight, I am fat and would guess I'm medically obese (fat around visceral organs). I looked far better at size 10-12 which, for me, is 8kg less than I weigh now.
I'm also tummy-prone and have been mistakenly thought pregnant. Now post-meno, I have the belly of a beer-drinking man but will continue with knowledge gained while fit, toned and hormonal.
Core strength can make a tremendous difference - but it needs to be very strong. The best exercises for this are slow and deliberate (Pilates) and need to be repeated often, with increments guided by a teacher. This teaching can specifically address persistent post-partum pelvic girdle separation, which is far more common than literature suggests as it doesn't need to be pathological to cause sagging and discomfort.
Building on the reliable core tone and flexibility gained with Pilates, the sweaty kind of exercise firms up the abs and other surrounding muscles. HIIT is indeed a great way to get a firm body (and start shifting some of the visceral lard!) Sweaty exercise is, regrettably, the only way to achieve this. It works - though I had to do it continually to keep my tummy muscles strong enough to automatically hold in the bulge.
Losing the extra weight was also necessary. I don't diet, so that was easier with the daily aerobic training using up my calories.
Also, don't underestimate the value of good shapewear - and, yes, choose clothes that fit you properly! Have them altered if, like me, you can't find off-the-peg things for your shape. (Being size 12 with a smaller tum really makes high street clothes fit better, though!)
Good luck 🙂 I've got to say life is easier when you care less, but I certainly understand and remember caring a lot about it.