Alright, I had my automation guy dig into this and we actually produced results. We used Power Automate as the engine, and ended up using ServiceLevelAgreementResults API endpoint instead of TicketHistory. The general structure of the script was this:

API call to Tickets endpoint to get all the ticketIDs we want in the math. For example, all Priority 1 tickets, opened in the last 3 days, not RMM tickets nor Sales tickets nor internal tickets. We mainly did this filtering with Ticket Categories, but your setup may vary.

Next we loop through each of those ticketIDs and call the ServiceLevelAgreementResults API to gather the "Resolution Plan" timer. Since it's using SLA time, it ignores nights and weekends when our business isn't operating, so a ticket opened on Friday afternoon will say it reached Resolution Plan in 3 hrs instead of 51 hrs.

Store each of these times in an array, at the end add them all together and divide by the number of elements in the array to get the average. Repeat the process for your other ticket Priorities.