gyro, did you ever look into putting maps.me on your smartphone?
Ive used this and used ODM open source maps, for the areas I am biking through, so at least you have the detailed maps and the phone gps can show you where you are. I used this on a tablet, and did not use it as turn by turn instructions, but only as a help when I wasnt sure where i was.
down load the app, then zoom into the start of your trip, and it will see that you need map info for that area, and it will ask , do you want to download it? Hit yes, wait, then scroll along until you get to end of that specific map info, repeat.
I did this for our Eurovelo 6 trip from Nantes to Switzerland, took a little while but when done I had all the maps.
I know that eurovelo routes have gpx files that you can find on probably the eurovelo website, but I wasnt smart enough or took the time to figure out how to incorporate these into my phone and tablet, but it should be doable . I just didnt take the time to figure it out.
ev6 signage was pretty good in France, although I'll be honest and mention that we missed the route a number of times, and I suspect in eastern europe it will less organized signs wise, so a clear advantage to spend time figuring out this tech stuff.
I'm still someone who does not have an interest to have a device on all the time doing the turn by turn thing, there is a real battery issue with this in any case, so if you do, you have to be realistic about this part of it.
good luck sorting it out, come back with info you get please and how you did it