HTTP sitewide, except for: wp-admin, and 2 custom directories

This is a pointless question. you either do a sitewide HTTPS or just don’t do it at all. Doing HTTPS for only some pages is just lying to yourself about the security of the site.

You can decide to have HTTPS for only some of your users, but you can never do it just on some of your pages without breaking the fundamental assumptions behind using HTTPS as a security measure.

If for some reason you need those pages HTTPS but just can’t do the whole site, then put those pages under a different domain/subdomain.