> Spacecraft could theoretically be used to extend the baselines but the volume of data to be transferred is prohibitive with current spacecraft comm tech.
Space-based very long baseline interferometry (VLBI) has been done at lower frequencies, most recently with the RadioAstron satellite[0]. There hasn't yet been a VLBI satellite observing at the same frequencies that the EHT uses, but there are mission concepts being discussed. [1] discusses some of the technical challenges.