So a friend of mine does a lot more DMing than I do and they asked me if I knew about a digital battle map where they could have movable tokens to keep track of effects, position, terrain, and HP more easily. We talked a bit about how it should function and agreed on this basic functionality:

• Square grid with the ability to colorize squares to draw terrain, walls, obstacles, etc. It's 100x100 squares, we agreed that'd be enough.
• Movable tokens with name, HP, and initiative attributes.
• A sidebar with an overview of the tokens sorted by their initiative.
• Automatic handling of effect duration. So if you correctly press "next turn" every time, it should automatically count down the duration of every token's effects.
• Save and load the full map state in case a battle is split between sessions or you want to prepare the battlefield beforehand.
• Tablet support.

I incorporated this and thought that maybe some of you might either have recommendations for a system that is better than what I've made here, or want to use it too.

It's currently in a state where my friend and I agree it's all we personally need, so I probably won't be adding more features in the near future.

Edit: I'm aware that it doesn't properly scale on smartphones. That's because it's not intended for smartphone use and I can't be bothered to implement proper smartphone scaling because it would be huge hassle and neither I nor my friend would use it.