BuddyPress.org

You can’t do that in PHP. Well, actually you can – but you will break all the styling, as you will filter our header/footer.php files include. So basically it’s not a good idea.

The best idea – to use a child theme. You can than even switch to that child theme for specific pages on your site. One of the ways to do that: https://premium.wpmudev.org/blog/wordpress-different-themes-for-pages/

Another idea – do some magic with BuddyPress templates. It’s totally possible, but you will quite heavily modify them (that will complicate future maintainability).

Also it would be easier to help you if you provided a URL to your site.

You don’t need a BuddyPress theme, any WordPress theme will work just fine.
You can create a child theme for your current theme and modify BuddyPress behaviour through it. Copy /plugins/buddypress/bp-templates/bp-legacy/buddypress/ to your theme (in root).

Remove everything that you don’t need, and leave only /your-theme/buddypress/members/single directory. In that dir you should work with home.php files and others, that are relevant to you.

But now I see that redefining BuddyPress template only won’t work as expected. You also need to create page-members.php file, where members – is the slug of you Members Directory page. It should be modified with

if ( bp_is_user() ) {
    get_header('slim');
} else {
    get_header();
}

and same for get_footer('slim') instead of the default usage. You will need to create header-slim.php and footer-slim.php files as well, and THERE you should remove all your navigation and all markup you don’t want that pages to have.

So yeah, quite a work, but doable once you dive into that. Hopefully I gave you the direction and general idea.