I must've searched for 30mins before coming across this, it's the EXACT type of tool I was looking for. Most of the digital tools out there are built for online sessions, or only include half of the equation. Previously I would have to reference the monster sheet, mentally notate the die + modifiers for their attack, switch tabs to a dice roller, input the dice data, then roll. And then utilize a 3rd tab to adjust HP/view initiative. Not to mention savings throws, conditions, etc.
Is this still something your actively working on? The ability to create/add custom monsters would make this complete. Often have to buff/nerf certain monsters to match the party, and have many custom monsters of my own.
I feel as if there's quite a few DM's out there that would really love this tool, and I personally would be more than willing to contribute a small amount ($10-20) towards the time you've put into it, and feel others would as well.