If you're going for more naturalistic transitions instead of a blocky, clearly-tiled look, you shouldn't use such straight transitions. Have bits going in and out, and don't be afraid to let things go all the way to the opposite edge sometimes. If you just have a straight line boundary in your transition tiles, that's only slightly better than not using transition tiles at all.
Also, try not to repeat parts of the other tiles in your transition tiles, as that only undoes the purpose of using multiple tiles. Take the time to make new grass/dirt bits that tile with the others. It's not that much more work, but it looks way better than repeating elements.
Here is a pixel-over where I only changed your side transition tiles, to give you an idea of what you can do:
It's two tiles total (plus horizontally flipped versions), same as yours.
If you're clever about the exact shapes and transitions between the tiles, you could even avoid having dedicated corner tiles and instead have "side" tiles that can be used for corners too. Making multipurpose tiles like that can make it easier to make varied, natural-looking maps with the same number of tiles, or even fewer tiles. Here is a somewhat extreme
example using 16 8x8 tiles (zoomed in and with a grid on the example usage), where almost every transition tile can be used as a corner or side, and a few can even be used for both horizontal and vertical transitions or as "islands". With 32x32 and with transitions between more distinctly coloured terrains it'll be much harder to achieve that degree of flexibility, but you can still make something much more flexible than what you have now.
