The best way (after endless tries of different methods) to adjust for a full tummy is the slash and pivot method in the centre front (crotch seam) larger tummy does not need extra width often but extra length - more fabric to go up and over the tummy, raising the waistband so that it sits level when wearing (this does not look level when not being worn, but that's ok
Also by using the slash and pivot method, the side seams remain the same to match the back of the garment so no fiddling about needed there.
Decide how much adjustment you need, you can get this measurement from a pair of trousers you already have that don't fit that well over the tummy area, stand sideways and look at the waistline of the trousers, it probably sits ok at the back but sits lower at the front, the difference from where the waistband is and where it should sit is how much you need to adjust the front in height, but adding it onto the top seam is not the way to do it, slash the pattern across from the middle of the crotch seam over to the side seam and stop a tiny distance from the edge ( put some tape here strengthens the 'pivot', then spread the gap until the desired additional height is reached (if there is a large amount, try making several slashes instead of one)
This will make the front of the trouser higher, which is fine. Redraw any darts on the pattern
if more width is needed (after trying the above) then add that by opening up the piece from the waistband to almost the bottom of the crotch to add more width
Make a toile without question, and if more adjustments are necessary, use the toile and add fabric rather than making a new toile, then use the toile as your pattern piece, or use it to adjust the pattern piece further
I tend to use the pattern size for the back, and adjust the front, because the back needs to be flatter
The problem with adjusting RTW, is there is not always enough fabric and only width if there is and nothing to add to the height
Also RTW have jodpur hips
This method works for me, I first learned it with Barbara Deckert (book and craftsy) and later also watched Kathleen Cheetham do it in craftsy, current searches where some of the newer pattern makers have tutorials are not, imho good enough explanations