Wow, am I glad I found this post. Since the upgrade to iPadOS 15 I’ve been feeling really alone in this issue. I searched a lot online, but somehow missed this post until now. I have the exact same issue.
For what it’s worth, I will share my experience with Apple Support regarding this issue here. I had an Apple engineer with whom I could share my screen, so he could see the issue for himself. He never saw anything like this, he said. We first deleted the Mail app, restarted the iPad and installed the Mail app again. No dice, still crashing. Then, on request of Apple Support, I wiped the iPad completely, to start from scratch, as if I had bought a new iPad. After this, I only configured the e-mail account. And everything went fine, the crashes were completely gone. Hence, concluded Apple Support, the issue is not iPadOS related, but there must be one of the apps I have installed that’s causing the crashing. And he left me with a ‘Good luck!’.
Since I didn’t want to download and (especially) configure everything again, I restored the latest backup of my iPad; and, of course, the Mail issue was back. I’ve been using the Gmail app since then. But I really hate it and would like to be able to use Apple Mail, as I’ve been doing since 2010.