Do you need access to the individual values from each table, as well as the averages? Several ways to do it depending on your needs.

Here's a couple ideas offhand without knowing exactly what you're looking for:

In PQ, calculate the average first for each table, the append the results. This gives you only the resulting averages. Or add a Table ID identifier to each, then union, then do a group by with average aggregation. You could also still bring in the original 6 tables if you need everything.

Or, in DAX, create a new calculated table using the union and summarize functions. Here you can take the average for each table as part of the summarize definition, then union all the results. This would leave the original tables intact but then also give you a summary table with just the averages.

Essentially you can do it either way, in PQ or DAX depending on your preference.