I will say Mae is probably my favorite, but I love a one syllable name!
Marty and Marty both work very directly, but they aren’t my personal favorites. If you’re looking for a very obvious nickname, though, they’re the way to go!
I think you could easily get away with Maisie, and it’s so sweet. While Molly doesn’t come from Martha, it familiar enough on its own that it could work, though there’s also Mel / Mellie from M. El.
Emmy is also sweet and makes sense! In that vein, you may also like Mimi.
If you like Hattie, I would probably stick with Mattie. I think the connection is easily lost and Martha and Hattie could absolutely be sisters vs. the same person, though I love Hattie as a name!