PHP 8.5 is pretty bleeding edge and released a few days ago, usually it takes some weeks to find the new bugs and holes in a major version, i'd recommend waiting a few months to use it in production.
Given XenForo's last bug fix update was last July, almost 5 months ago, I'm pretty sure core XF is not yet compatible with PHP 8.5.
They even only recommend 8.3 for full support.
Yep. IIRC some vendor dependencies are not fully compatible with 8.4+ and can't be upgraded as that would break compatibility with 7.2.
Therefore I'd stick with 8.3 for now, this PHP version is still fully supported and will receive security files until december 2027.