Power BI taking ages to apply query changes

it's quite a journey before you :smileywink:

this topic has lots of sources:
https://community.powerbi.com/t5/Desktop/Good-Source-to-Learn-DAX/m-p/537748

here is an example for the cumulative totals:

https://www.daxpatterns.com/cumulative-total/

regarding your particular model it would be better to revisit once you learn the basic concepts, especially if you come from SQL background - DAX has very different way of solving same problems

if you're under time pressure though here are few tips:

1) instead of quarters as text I'd use dates (e.g. 2019-01-01 for 2019Q1, 2019-04-01 for 2019Q2). This enables using more operators, as well as gives access to the time intelligence functions

2) build a separate Calendar (date dimension) table (for the benefit of time intelligence):

https://www.sqlbi.com/articles/automatic-time-intelligence-in-power-bi/

https://docs.microsoft.com/en-us/power-bi/desktop-date-tables

3) if the baseline scenario is just prior year figures you could calculate these values based on Actual & Calendar table (e.g. using SAMEPERIODLASTYEAR)
https://docs.microsoft.com/en-us/dax/sameperiodlastyear-function-dax