How to debug a transmitting configuration?
I figured it out. In a very ambiguous part of the CC1101 datasheet it is mentioned that preamble settings will be ignored for certain combinations of settings. Once I fixed that and switched to a manual transmission of a preamble, everything worked fine.

For future reference, these are things to try when debugging TX:


Use a spectrum analyser and oscilloscope to confirm your timing
Make your transmitter and receiver emit a continuous wave, and make sure your frequencies match up.
Check whether the length of your transmission matches your expectation.
Check the RSSI for your communication. Does it make sense?


Thanks for thinking along, everyone!