I doubt it. Kate started with very very limited exposure and has only in recent years upped her working game. She also isn’t the best public speaker (no shame in that by the way) but it is pretty obvious she got training and has improved quite a bit over the years. If they had taken years to prepare her, I think she would have started into her royal life differently. I do think they have learned from Diana and gave Kate and Wiliam time to settle into married life with a more or less „normal“ life with very little royal duties. But obviously Wiliam‘s role was and still is very different to Charles‘ when he married. Having such a long reigning Queen definitely worked out for them.
I still wonder if the whole pressure of “hitting the game running” (aka media accomplished, working high numbers straight away aka giving Kate a run for her money) was pushed by M&H because BP should have known that the transition is extremely hard (+ not having friends and family nearby and being in a new country). That was so unnecessary. Yes, people expect more from someone ten years older but they could have eased her in more. This narrative broke her back pretty early on. She really did a lot but nobody could have lived up to those expectations. Especially if you are very clueless about the life you are entering. Not knowing how to curtsy? And that they still do it in private? That’s actually very basic knowledge if you do some research. So yes, she would have needed more time to learn the ropes and maybe less engagements straight away. Might have made a ton of difference.