The election of chairman at the annual meeting should be a single agenda item, and come first on the agenda. The legislation provides the potential for the declaration to be signed at a later date, but only by resolution, which would need to be recorded in the minutes, so in my view, the minutes should state clearly whether it has been signed at that moment or will be signed later.
I always pause the meeting to obtain the signature at that time, for both new councillors and election of Chair, and record this fact in the minutes.