The OP has probably since moved on to other things but actually there is a solution to this problem right here on the forums as of August:
And here’s a link to the Github:
The technique is an AI based technology called ‘Semantic Segmentation’. It’s the same technology found in Photoshop’s smart object tool and there are even some Apache 2.0 licensed versions.
In Windows it’s very easy to install the addon, in Linux it can be a bit harder; but the guide works.
It is a remarkable tool and I have personally tested it on windows and Linux and found it it be exactly what you’re asking for.
Happy Krita painting to anyone stumbling upon this.
Kind regards,
Simon.