Glad to help you all
(but dont forget the thanks button is there for a reason)
The app mentioned is definitely the one needed. The reason is because it has that specific interpolation method "Lanczos 3". Im willing to bet that the resize/image editor apps dont have/use that type of interpolation technique.
As far as collages, I would suggest making the collage first, saving, importing to "resize" app with settings adjusted as mentioned in The Verge post (interpolation and image size changes), saving, and upload from gallery to instagram.