Tall horses don't jump better so going up to a taller horse isn't a 'plus' especially as they take more to hold together especially for a less experienced rider than something more compact and 'punchy'
I'd move away from the TB's and look at some cross breeds that have more depth of girth and build about them. There are some good quality draft x TB's about, the Irish Draft x TB had always held its own in the jumping ring at top level. We have a WB X TB that's only 16'1 but she's a really well built mare and looks way bigger than she is because she has so much body
My son still rides his 15.2 cob and is the same height and weight as yours and one of my other sons was still tucking his legs up on a 14.2 UK jumping pony at 6ft so he could get another season in junior pony classes