My dd had simular issues between 6 and 8, all piled on during lockdown. Our saving graces were Tu, the plus size range - I'd buy right up to 10/11 in shorts and cropped leggings and it would work OK. If the waistband is still digging in then you could cut through it and put a couple of stitches below the cut to stop it tearing? I did this with knickers once for her out of sheer desperation.
Their dresses too fitted well, again sized right up. Obviously the smocked and swing designs, nothing more slim cut.
I also got lots of boxy t-shirts from asda which looked fantastic and were really comfy. Super cheap too.
Jumper dresses from H&M were a surprise hit, she loved them. We found some nice ones in River Island this year too. Those with a pair of knee high boots made her feel the bees knees. Tights are tricky but I used to size up for the waist and just cut the feet off so they wouldn't wrinkle.
I found gap hoodies were a good fit, and I don't know how easy they are to find in the UK, but American Eagle ladies XS sweatshirts are surprisingly not ridiculously long in the arms. They fit the current oversized look well.
The only school coats I found were a Jasper Conran one I found in the sale and an Oshkosh one I got an Amazon.com. Neither very cheap but they were both lovely coats.
Check out the charity shops. Buy things you think might work and have a go at altering them, you won't feel so bad if it all goes wrong! If you're not a great sewer use bondaweb, the iron on stuff. That's where I discovered the H&M dresses and the gap hoodies.
Good luck with everything, it's such a difficult thing to tackle. It broke my heart when my daughter complained about her big tummy. Her appetite naturally waned as she got older and life got busy again, and now she's back wearing clothes that match her age without the need for a 'diet' as such. It was an intensely worrying time though and I wish you both well.