Hi Markus!
Great to see you here, and sorry I've not chimed in earlier but I see the forums folks have been rather helpful/informative so far 
I can clarify the Foundation situation a bit. (And we absolutely share the pain you feel
The foundation team actively working on open sourcing a "Foundation implemented in Swift" which then will be the same across all platforms. That plan was shared here in some more detail: What’s next for Foundation and all I can add to that is that the work is still actively underway 
It's been a long time coming thing and we're very happy to finally properly solve this annoyance that you've rightfully pointed out here.
Until then you sadly cannot "pick" the open source one while developing on macos.
For what it's worth for server side projects we often simply default to not using Foundation "as a whole" but e.g. specifically importing just Date, or JSONEncoder etc. Which may make this adventure also easier for you. I'm more than happy to help with specific trouble you hit and we sometimes have foundation-free solutions to things, feel free to ping me about any needs you hit.