That would probably work, but I have not explored the datamart feature much.
What I ended up doing is exporting the dataflow and adding a filter for the PreviousNMonths(2) to the tables I wanted to filter and set up a new QA dataflow that I only need to refresh once a week (or month, if MS gave us the option) in Notepad++. Tried doing it in the PQ Online in the PBI Service and it was taking forever, so I just edited the JSON manually. Took 15 minutes to filter 30 or so really large tables to a workable size for me to build datasets from. I can then reconnect them in Desktop PBI to the full dataflow after I have the dataset built.
PBI is such garbage. My old employer used Tableau and I miss it so much. Tableau wouldn't even blink at a 1m row table, much less need an hour to join ("merge" WTF) it to another.
Thank you for the idea, though! I appreciate the time and brainpower you gave me.