His wife is herself busy with her sailor moon manga.If his back gives him so much pain he should collaborate with his wife/some othere mangaka.He would think of agreat story like he always does while the other person would do the drawing. In a fact I think that his manga would look visually better thanks to this because his art/drawing style is not of particulary high quality to be fair. Especially compared to kubo,oda or kishimoto
And his art is not bad he is just doesn't do it under stress, let me show 2 examples:-
1.Look at the turkey in this picture and see the level of detail then see the scribbling art:-
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2.This is from the latest 9 chapters he released.
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As for assistants he never keeps them and is stubborn in that regard.
Also his art tell us when he is going to take a hiatus, whenever quality deteriorates that means he is going on a hiatus soon.