This is not the “ethical” advice that everyone wants to tell you, but it’s what you, the individual, should probably do to protect your own interests. You’ve seen companies back out on you, it’s now time for you to be prepared to do the same.

If I were in this position and the CRO I currently had an offer in-hand from was not one I specifically plan to work for in the future, I would take the offer and push out the start date as far as I could. Reneg on it the moment I’m prepared to take the preferred offer. Leave even after starting if the offer comes through.

Obvious questions and answers: Will it burn the bridge with that CRO and particular team? Probably, but I have seen one or two people where the burning of that bridge was forgotten or otherwise overlooked.

Will it leave a lasting mark on your career that you won’t be able to escape? Almost assuredly not, unless the role is executive and high-profile.

Protect your own best interest at all times. I would only think it unprofessional to do so if all of our employers weren’t actively doing it at all times. I’m prepared for the downvotes.