what I found a practical way in case of tiles, to have first a map filled with a default 24b tile (or have a 24b base image in case of using terrain as a base), and then put the 32b tiles having borders with alpha fade out for good blending with the images placed previously.

As a better and more randomized approach, later I used a separate grey-scale image storing the alpha fade out values, and used large texture files instead of tiles. Their repetition is dependent on their size, so can be different for each texture, while you can also change randomly the alpha fade out image - if you make more versions.

It was my first trial project with 3DGS I used it for tile aligned painting of terrain texture (using a multi-terrain system, so neighbouring terrains were also painted), plus during painting tile settings are also can be managed. You need an editor for this, or create randomly.