Numbers data sheet for auto complete

Wait, there may be a better way. A separate table CAN be the source of all the "previous entries", as you suggested. This way you can add items to and delete items from the one table and it will apply to all your other tables.

As in the first solution, insert a bunch of new rows at the top of your tables, directly below the header rows(s).

In a new table will be a list of entries.

The "bunch of new rows" you added will reference each of those entries.

Here is a quick mockup.

Table 2 needs to have the same number of header rows as your monthly tables.

The formula in B2 is =Table 2::A&"" (Use the column "A" reference not cell reference "A2")

Fill down to capture all rows of Table 2

Hide the new rows (which I did not do for the screenshot)

You can see in cell B12 I started typing a G and it is giving me an autocomplete list

That would be interesting and useful but it is not how autocomplete works. It makes its list from the entries already made in the column. I am not aware of a way to do autocomplete based on the entries in a different table.

You could insert a bunch of new rows into each table and put all those possible entries into those rows, with no data in the other columns that would affect the results of your table and calculations, then hide those rows from view. I would put them at the top (directly below the header row/rows) if you insert new rows to the bottom of your table as the month progresses.