Random ideas:

• Caching of the main feed, so pressing “back” after being on a post doesn’t shift things around
• Caching of the last post, so going back to the main feed then opening the post again does not reload it

Those two might only work with an SPA or possibly the PWA.

• A button to hide read posts
• Gestures on mobile (swipe on a comment to vote or reply, maybe swipe on the post to go back to the main feed, though that might need an SPA)
• I know this is difficult, but maybe infinite scroll