mental illness can be seen with 1st house or 6th house moon, hard aspects to uranus, moon with north node (this is in vedic, but it works just as well with tropical as an indicator)typically if you want to look for mental illness you look at the state of both the moon and the 6th house before anything else
i would argue 12th house sun or venus is far more likely to tend towards depression and addiction same with hard aspects between the sun and neptune(though this can be a factor in an illness like that, it would be that side of it you were reading when you looked at the 12th)
and easy aspects shouldn't be ignored as potential issues
my own chart is a good example of this with a 12th house sun trine neptune and square saturn
the easy aspect to neptune becomes an outlet for the hard aspect to saturn and as a result the tendency to abuse substances is indicated in my chart in those 2 areas
venus along with jupiter can indicate issues too
you would also want to check out the state of mercury, and honestly all areas of a chart
typically someone with any severe issue is going to have it all over their chart
because there isnt a single aspect
those ones are just the better starting points to look for what disorder fits
medical astrology tends to require looking at every detail to narrow down specific disorders
the state of someone's mars can reflect a lot as well since mental issues manifest as actions and arent limited to just the mind but the way people act in the world around them
complex issues manifest in many different areas, so any single indicator isnt going to do it, but when there's a lot spread to most areas of the chart you can see more of a pattern and diagnose something like that