I agree. It's the "where do I start?" syndrome. So much to correct, so much to advise, but at the same time you are reluctant to ground someone that had no idea when they came to you that they had so many bad habits or gaps in knowledge/skill.
In a good club, there should be ample opportunities for refresher training, e.g. in a series of workshops combined with flying. But of course you can't force people to come, and it's often the ones that don't really need it that do come. Plus, even the ones that know they need it are very often loath to admit their need, especially in front of other club members.
And this does nothing for the owner-pilots who are not part of any club.