As I said, I thought about it, but I did not do it so far. So it's all guesses.
Those 'archive_day_somthing' tables are derived tables from the archive table. As I understood the WeeWX architecture you need a table like the 'archive' table.
In the first place I would try to put all those values into one single year and refer them by the $span tag.
Another possibility would be to put the values into an config file and provide them in WeeWX by an searchlist extension or XType extension. That requires some lines of Python.