IIRC the addition of the hydroxy to the drug means it's going to be more water-soluble so it would be easier to excrete in urine and flush it out of the system, which would count as "less toxic" if you squint when reading the paper

I haven't seen any detailed discussions of the differences in toxicity, but one physician made a comment to that effect.

You probably don't have to derive the relative toxicities axiomatically, since volumes appear to have been written about the lower toxicity of HCQ compared to CQ.