Couldn't they do it like they do for the recent Call of Duty games? In those games you can chose which parts of the main game you have installed...
I had the same issue today on Dresden. I've had it on some other routes too like South Eastern High Speed.
So I was having the same problem with Dresden not showing up and slow loading on PS5 earlier in the week. I have all the route DLCs except Cane...
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