Thank you both for pointing me there! I guess there was wishful thinking involved on my side then. Using USB MIDI without a computer seems to be a niche use case. I think that made it hard for me to find a source to understand how it actually works that did not start with expecting me to want to connect devices to a DAW. Those options do look interesting though.
The RK-006 seems to be made especially for this use case, though I am a bit hesitant with a small box of plastic and getting into those TRS MIDI cables. It just does not feel sturdy and reliable from a first look having these flying around behind the scenes with cables going in all directions. Also I could not find any information about latency and how to filter/distribute virtual USB ports internally other than everything that goes in will be distributed to all outputs. I am not sure if that is what I really want or if it will lead to loops and limiting the channels that can be used.
From the manula for the Mio it is not clear to me, if it really will work as a stand alone MIDI host. At least they don’t expect someone to want this. Otherwise it looks great and might be something to go back to later when the issue, that is now more of an inconvenience gets more urgent.
There also is the upcoming conductive labs MRCC, wich looks great and can be preordered now with expected delivery in March. Standalone desk device though, with a clumsy rack ear option, but very intuitive routing and filter options!
Probably most people who are interested in something like this work on a desk (or floor
) and the MRCC design with easy accessible jacks on top/front is great. While those who want to have something racked with jacks on the back most likely just want a MIDI interface and connect it to their computer anyway.
I am centered around a Circlon and only connect the MacBook if I want to seriously record something. So I went for a simple Kenton through box and USB power supplies where needed and just leave the USB only devices out for now, respectively keep them connected to Norns, which works as USB host, as I understood now - so I should be able to to connect it to the Circlon and have them work or at least synced together. I guess Norns will not automatically distribute MIDI information to other connected devices, right? 
I suppose in the end the main problem here is a collection of interesting little thingies, soldering projects and standalone boxes, growing around a modular system without having a proper designed studio backbone to carry all that. Plus not wanting to use a computer out of mere practical reasons. When I have to connect the MacBook first and start up Logic, there is an inhibition threshold holding me back from making music. When it is connected, there is the same mechanism holding me back from doing my payed work - I am annoyingly complicated with actually doing my stuff…might just need a second computer. The new MacBooks look great but will probably neither connect to my audio interface, nor host the windows application I nned for work).