The example animation is not optimised and is already in RGB mode ? but for any other, check those first.
To erase an area as your example screenshot, one way is using two scripts (1) sg-combine-bg.scm to paint in the area for erase, (2) c2a-all-layers.scm to erase a color.
The procedure is
Make a layer at the bottom of the stack with a color as the area for erase. Use the pencil tool for hard edges, no anti-aliasing.
Apply Filters > Animation > Overlay Background (script 1) Then delete the bottom layer.
Apply Image > color-to-alpha-All-layers (script 2) Colour pick the painted in color and OK to remove the color.
That is your intention so far. You could of course just use (1) to paint over in black without that transparent hole
or
To fill in the hole, another layer at the bottom then Filters > Animation > Combine Background (another part of script 1)
Sorry this is English commentary but you might get the idea https://youtu.be/EG42e_TPVI0
If it is something else, give details.
Two scripts attached. Unzip, put in C:\Users\"yourname"\AppData\Roaming\GIMP\2.10\scripts
To erase an area as your example screenshot, one way is using two scripts (1) sg-combine-bg.scm to paint in the area for erase, (2) c2a-all-layers.scm to erase a color.
The procedure is
Make a layer at the bottom of the stack with a color as the area for erase. Use the pencil tool for hard edges, no anti-aliasing.
Apply Filters > Animation > Overlay Background (script 1) Then delete the bottom layer.
Apply Image > color-to-alpha-All-layers (script 2) Colour pick the painted in color and OK to remove the color.
That is your intention so far. You could of course just use (1) to paint over in black without that transparent hole
or
To fill in the hole, another layer at the bottom then Filters > Animation > Combine Background (another part of script 1)
Sorry this is English commentary but you might get the idea https://youtu.be/EG42e_TPVI0
If it is something else, give details.
Two scripts attached. Unzip, put in C:\Users\"yourname"\AppData\Roaming\GIMP\2.10\scripts