For UI consistency, tapping the Feed button after you’re already in the Feed View should scroll to the top of the feed. That’s what similar apps do (like tapping the News button in Apple News).
If you’re worried about accidental taps, pressing it a second time should take you back to where you were in the feed.
The other toolbar buttons should work similarly. If you’re in the Inbox and you tap a comment, tapping Inbox should take you back to the main inbox. Tapping Inbox when you’re already in the inbox should scroll to the top of the Inbox (if you’re not there already).
I’ve just had the quickest of glances through the code. Is the gesture for opening the user switcher the only reason for reimplementing the tab bar? It sound’s like you’re rebuilding a lot of functionality that you get for free (iirc) with the native tab bar.
Unfortunately the system (SwiftUI) tab bar doesn’t come with this behaviour for some reason.