A general rule of thumb - is the individual Cllr affected / impacted / advantaged / disadvantaged MORE than the average resident of the wider community.
If yes, then there “may” be a case for declaring an interest but keep in mind that Cllrs are of and from the community so a significant event which impacts upon the broader community, may inextricably impact upon a Cllr BECAUSE they are of and from the community.
Anyway - the decision as to whether, or not, to declare an interest lies solely with the individual. Anyone else’s opinion is irrelevant unless after the event the details are highlighted as a CoC complaint for MO adjudication and we have plenty of examples of the ineffectiveness of the MO.
I personally submitted a CoC complaint about a Cllr that failed to declare an interest for a housing estate immediately adjacent to his declared residence and the MO declared that it wasn’t upheld because it was an outline rather than a full planning application - what a load of tosh.