If you have the time, I’d also love to read a blog post (or even series) explaining how you built this. Your answer on the Graphhopper forum was very clear and makes me think that a more detailed version could be super useful for a lot of people.
Nice work!
Subscribed to you on twitter to get updated if you publish a post on that!
The thing on absolutely excluding – yes, maybe some things are impossible, but in my planning, it’s really about the journey, so even if I have to go additional hundreds of kilometers, so even in terms of extra days, but via cycling ways and tiny country-side streets (without max speeds over 30km ideally or similar) and not see fast moving cars almost at all, which in Berlin/Brandenburg area, for example, seems to really be possible if you plan it manually. Judging by the success of applying your rules on the trails it surely can be done better than I have seen so far.
I'll probably write a blog post on how it's built though - there's quite a lot going on under the hood!
Supporting cycling is a possibility for the future. I don't think you'd want to absolutely exclude non-cycleways (as it might make many routes impossible), but you could certainly weight very heavily against them and show on the map which parts of the route were dedicated to cyclists vs which were not.