Tiling over old 2x6 tongue and groove subfloor. Underlayment? Levelling?
I quite literally did this exact same thing a few years ago.

First ensure that all of your structural posts have support beneath the house to foundation.

Then I got decking screws and screwed the planks down everywhere they were already nailed.

Then I bought about 50 sheets of CCX ply and lots of subfloor glue. I glued and nailed ply over the entire floor using shanked dipped nails.

Now I had a very stout floor about 2" thick....

Ditra over that, tile over that.