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Count items in one list and return them in another

I am working on a flow where a program owner sees a list of applicants for the program and when the program owner assigns the applicant to a session (3-01 cohort in the example below), the flow looks at all the classes in the Class Schedules & Rosters list that are cohort 3-01 and creates each in the Class Assignments list that we use for rosters.

The list I'm using the titled "Class Schedules & Rosters" uses the following columns:

FilledSeats (CountRelated LookUp of the Class Assignments list)
SeatCount (Number column)
Seats_Available (Number column)
Course Title (Look up of the course title from the Course Title list)

I'm able to do everything in the flow BUT rather than using the FilledSeats (CountRelated LookUp) I'm trying to automate the filled seat count so that when a new class assignment is added it reduces the Seats_Avail column by 1. Additionally when a learner moves from one session to another their count is subtracted from the session their leaving and added to the session they going to.

I'm trying to stay away from lookups too but beyond using a calculated column to bring in how many seats are taken, I'm not sure how to do what I want. The other issue too is that because the Filled Seats is calculated AND a count related, I can't get it, or find it when trying to pull it into a Power Automate.

ASK: Using Power Automate is there a way to create a flow where the list item is created or modified you get a count of how many learners are enrolled in each distinct class in the class assignments list and then add that number to that class in the Class Schedule and Rosters List?

Scenario: If the same class is listed say, 10 times for 10 different learners, that the Seats_Available column is the SeatsCount - Seats assigned in the class assignments list?