Welcome! This forum has a treasure trove of great info – Scouters helping Scouters! Just a heads up, though - all content, information, and opinions shared on this forum are those of the author, not the BSA.
I believe that the Unit Secretary functional role in Scoutbook can still edit calendars, no? They need at least View Profile/View Advancement permissions for all scouts to send messages.
Also keep in mind that the Committee Secretary functional role may not be automatically assigned connections to new scouts, so a unit admin will probably need to update connections regularly (or at least whenever a new scout is added) to ensure that the individual has connections to all of the youth.