Turned out I needed to complete the War Campaing on Zandalar. Then I was able to start a quest “To the Front” at the War Table on Boralus, meaning I finally could see the portal to Arathi.
So, apparently there’s a requirement to travel to New Arathi (complete War Campaing on Zandalar), but there isn’t any to travel to New Darkshore.
Anyways, I solved it and everything’s cool now.