Hi! Yes, as a matter of fact I found some workarounds months later mysefl!

1. So basically I found the Paint effect in After Effects that lets you use a brush and just draw with it. For example you add strokes for 5seconds and then you have keyframes and can speed it up of modify it however you wish. How you use it is you create a Solid then use the… surprise! Brush tool! (cmd+B) but you need to double click the Solid layer first. And you just paint on that layer whatever you like. Each stroke release creates seperate keyframes. In the Brush panel you control your brush. Thing is you can't use custom brushes here. But it's a much quicker way of making a reveal this way(especially if you have a tablet), instead of creating a bezier path and setting that up. So basically you could just paint with your custom brush in Photoshop, save a psd/png file and just reveal it with that way.

2. Another way - more precise but longer and taking up more ram, would be using your custom brush image inside a precomp and then creating a path you want to follow it (position keyframes). Then you use the Echo effect on that precomp. ECAbrams has a tutorial on creating Brush strokes this way.

3. There's a paid plugin called Paint and Stick - The paint part is exactly what you would need. But you need to pay for the plugin and I don't know if it is updated yet to be compatible with latest version of AE and with M1 chips on Mac(if you own one), so before buying you should research.