I think the 'mistake' was setting 10 Matchdays when some matchdays are 2 weeks of games apart. You can still increase Matchdays but yes Pot B leaders will be unhappy lol
And rightly so should they be, because implementing additional or extra match days now would be highly unfair to them, maybe extra options to stake on might not be as unfair because the possibilities of that was mentioned before the beginning or at the early stages into the pool kickoff.
Since each Match days was determined by the official game rounds as it also is in the pot A, the interval and how long it takes for one match day to get concluded is irrelevant and not a mistake imo, more so it's just the maiden season of the pot B, so there is no way to get it all near perfect, however, I think it should get even better as it progresses, especially with how widely received the new format is.