There are google sheet add ons / scripts you can use for each of these options, or zapier / make.com (has the advantage of free tier) / n8n - which will probably look something like:
Check for missing critical data
Update action performed date stamps in google sheet
Google sheet formula calculates time from datestamp of action performed
Separate flow - triggered by calculated date for rep to check if meeting exists and follow up if not
The follow up part can also be constructed in different way - depending on the meeting tool / calendar you use, you could update a column with meeting booked Y/N and add that as a condition for the follow up date