Magic knickers - that's my answer.
I really, really sympathise with the OP. I too have had a saggy tummy after having my children and I went to my GP to find out what I could do and was told I have diastasis rectii (i.e knackered tummy muscles that have over-stretched and won't go back to where they should be.)
I've also lived with this for 10 years and have considered a TT on many occasions. Then, out of the blue last year I was diagnosed with breast cancer and had a mastectomy, so now I still have a fat tummy but also only one boob.
I'm 'entitled' on the NHS to a reconstruction and it might be possible to take extra flesh from my stomach to make a new boob. Brilliant, you might think, all problems solved in one go. Sounds great in theory, doesn't it?
BUT I have decided against it because I had such a horrendous time after my mastectomy, and that was such a smaller op (even though it was still classed as major surgery). I reacted badly to the GA, I kept blacking out, I couldn't talk properly for a week, I could hardly walk for several days and I had a seroma (build up of fluid) for 6 weeks which kept bursting (in bed and on one glorious occasion in the GP's surgery all over him!!) and needed dressing daily from the district nurses.
If I was that bad after this op, how on earth would I cope with having an even bigger op taking bits from one bit of me and adding them on to another?
Am I trying to put you off - you bet I am!
As I said, magic knickers are the answer, and in my case, a silicone boob.