Hey I got the tip to crosspost over here and finally got the time to do so. So what do you think? Is this something that might be added in the Voyager app eventually?

The post feeds are delivered server side, but Voyager supports multiple accounts if you want to tap over and see posts from another instance's local feed?

Yes, sort of, in the Voyager app, but it’s awkward and unintuitive.

Pretend you’re going to create a new account or log into a different server through Voyager’s interface. Search up the server you want (sh.itjust.works in your example). Tap it so there’s a checkmark on its entry in the list, but instead of pressing the big blue Next button at the bottom, tap on the three dots in a circle icon in the upper right, and choose “Connect as Guest”.

Then it will be one of the account list entries and you can switch to it like you would switch to another account. Go to the community list from there and select Local. Now do a very awkward dance if you find a post that you want to interact with from your logged-in account. Best way to use this “feature” is probably just as community discovery to then subscribe from your account.

No, you can't even get a list of every community on another instance to subscribe to all of them.

Nope

Nope. The best option you have is tapping on "All" if you want to view all instances' posts.

Liftoff had this feature, where you could log into multiple instances at once and they would all appear in local. Too bad it hasn't worked since version 0.19 and has been abandoned. 😮‍💨

You can get the RSS feed and check it in a feed reader, for example for Sh.itjust.works you can use the feed https://sh.itjust.works/feeds/local.xml?sort=Active .

I'm not aware of any Lemmy app having such a feature though.