Logic

I'm not sure if i got what you a trying to do but try the below steps. I had similar requirement and was able to get it solved with the below steps:

In Power Query Editor, select Table 1.
Click on Home > Merge Queries.
Select Table 2 as the table to merge with.
Choose the columns that are common between the two tables (the columns you want to compare).
Select the Left Anti join. This will return only the rows from Table 2 that do not have a matching row in Table 1 based on the selected columns.
After the merge, you will see a new column with a table icon. Click on the Expand button (two arrows) next to this column.
Select the three columns from Table 2 that you want to add to Table 1.
Go back to the main Power Query Editor window.
Append the non-matching rows from Table 2 to Table 1.
Use Home > Append Queries and append the expanded result to Table 1.
Close and Apply | This process will ensure that only the unique data from Table 2 is added to Table 1.