Was backing Yara and Theon over Euron Dany's worst mistake in hindsight?
Dany chose to support Yara and Theon over him simply because they arrived first. Despite Euron having just been elected King by the Ironborn lords. So he had no choice but to go to Cersei.
If she had accepted his offer, she likely could have delayed the wedding until after she took the Iron Throne and/or killed Euron at some point if she wished.
Without Euron's help, Cersei would have almost certainly fallen when Dany first arrived.