Hey!
It was a few months ago, I did try a few more things, but I can’t recall what exactly.
In my case, I have a digital twin of my hometown island in real size - 57km long x 21km wide… I know… That’s huge AF… 
And I can tell you that neither Unreal or my PC like me very much for trying to achieve this size of a digital twin as an open world.
The main issue is that this island is not flat, basically anywhere… And besides the 57km x 21km, the island would effectively have more than that, since we also wanted to add below sea level landscape data to it… so i would range from -120m to 1700m of elevation…
And this is why the tile by tile thing would work perfectly for me in such a giant landscape…
Although I can tell you, that I did not have any more time to spare to figure out how do it tile by tile…
Haven’t messed around with it since, but What I ended up doing - does not solve tile by tile, but does it on a major landscape instead - which worked good enough for my usecase at the time.
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I have several groups of tiles as one unique landscape and aligned them up… In total I had around 8 landscapes faking as tiles with tiles of their own…
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I moved each landscape to a streaming level of its own and used the Load / Unload stream level nodes in blueprints.
I know it’s not exactly what we want, and its definitely not recommended to do it, as I encountered a huge batallion of issues, but that’s how far I went with it.
I will re-visit this issue eventually, and will for sure post here If I ever find the solution for it.
But althoug this worked for me as a demo showcase for a few features, I do not recommend it at all since it has so much performance issues, as well as in everything else.
Sorry for the long anwser, just wanted to share everything I did, in order to see If it helps someone anyways.
Thanks for reaching out, I will have a look around and If I get the knowledge of how and if this is possible like we want to, I’ll reach out for you.
Many thanks,
See ya!