May I ask what combination of settings results in optimal integer scaling to 1440p from the original 480p resolution (scaling, fit to screen, etc.)? I've consulted the GitHub manual, but it's a bit unclear to me.
Your resolution is 2560 x 1440, right?
The original game was 640 x 480.
So a x3 scaling would give 1920 x 1440.
This would seem perfect for your 1440p resolution as you fill the full vertical resolution, and maintain the 4:3 aspect ratio with pillarboxing, but DevilutionX will also allow you to play widescreen, so should then fill the areas on either side.