I have nearly 19k tracks in my radio and I run into this issue multiple times a day, trust me. However I think that the real issue is not adding tracks one by one (as suggested in the post) but rather the absurd time it takes to fetch next radio track. It can range anywhere from a tolerable 1s to even a minute in some extreme cases. I haven't checked the implementation of the radio algorithm but from my experience I can say that there is space for improvements there.
Having that said, I think that the proposed solutions are just workarounds for the real problem however if we dump the fetching time to something below 2s, those proposals would be something that would enhance the UX so overall I think they're good ideas.
Adding tracks when scrolling is not a good idea in my opinion. Not only we'd have issues with current autoscroll or resizing but it would be just irritating to some people. What gcrk proposed seems like much better option.
What we could do to give users ability to fetch like 10 next tracks, is to redesign this notice
into something like that, that would be fixed at the bottom of the queue view rather than having it at the bottom of the scroll container: