A regional operator where the ticket can't be bought online or via a smartphone should be really really rare in Germany by now. Which operator is it, which has no online payment option?
...which I guess is why I never looked further and saw they have an app now. Never heard of Naveo before, maybe it was introduced recently? Either way, that looks like it might now be possible! Why did no driver ever mention / did the buses/stations not advertise this before it's ripe for being replaced with the 50 euro subscription?!
I use it sometimes for the last bit of my long distance travel if I visit my parents (not directly in VRR region but in Regionalverkehr Münsterland (RMV)). It states that it works for all stops within NRW - which is annoying as one of the busses I could take actually goes to Osnabrück (Niedersachsen) and I cannot use - even though it's actually operated by the NRW regional transit provider.
Overall public transit is / was a mess in germany and 9€ Ticket really was an eye opener on how easy it could be to hop on and off of everything that's moving however you like.