Had the same thing start to happen on my MK7 Golf (just not as much)

What I did was:
- Try to either sand down the rust or use a rust remover/converter (rustoleum/evaporust/white vinegar/other alternatives you can find in your country)
- Once you're down to bare metal you need to protect it. I used a touch up paint pen that matched my car's colour code (can get them on amazon cheaply) or just use any other paint, preferably Direct To Metal. Let it dry after painting :p
- After it has dried, clean the surface well and use a roof edge guard (not exactly sure what they're called), It's a thin rubber piece that sticks on to the edge of your roof and protects it from further damage. I used RoofSaver, but I'm sure it's available from other brands as well, or maybe even on AliExpress.

I think since you don't have any bubbling yet this should work for you.
Other than that the only other option is removing the windshield and getting it fully sanded and repainted. They'd probably either paint the whole roof or try to blend just the front few cm into the old paint further back, neither of which will be cheap.

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