Use a black-to-transparent gradient on your mask instead of a black-to-white one.
Another option would be to create a smaller, sharp Layer Mask and use the "Feather" in the Properties Panel to soften the edges.
A layer mask is a selection and you can save selections as alpha channels. Alpha Channel can be combined into new Alpha channels. So you can build your layer mask in stages if you want. Once you have create one of the required five layer mask you can unlink it from the layer content and save it as an Alpha channel named "Image 1" then transform the layer mask to the other four other locations and sizes. Saving alpha channels in each location named "Image 2" "Image 3" "Image 4" and "Image 5" . You can then delete the layer with the layers mask. For when you save the Background layer with the five Image Alpha channels as a PSD file. You have a collage template that can be easily populated. You can also add layers over the background layer that can embellish the collages with. For example add a mat layer with five image areas cut outs. Use the five Image alpha channels loaded as a combined selection then cutting the areas out image areas of a mat layer. Then add a texture and style to the mat layer.
A video showing a 5 image collage PSD template being populates with images Automated via a Photoshop script from my Photo Collage Toolkit for Photoshop. Documentation and Example