You only need to confirm that the council meets the criteria for general power of competence immediately after an election, not every year. It must be reconfirmed after the next full election, if applicable. The declaration stands even if elected members fall in the four year period.
Where a council does not meet the criteria for general power of competence immediately after an election but subsequently has an election (not a co-option) that results in a two thirds elected council, then the council can claim GPOC immediately. Similarly, if a clerk successfully completes CiLCA (or an acceptable qualification) and the elected numbers are no less than two thirds, GPOC can be claimed immediately thereafter.