I think you also need to keep in mind exactly how many areas of litigation exist. Depending on your keyword match types and negative usage, you could be landing on a lot more than you’re expecting to very easily with an ad group like you’ve described. On top of that, every user searching the short-tailed lawyer terms also has a specific need in mind and won’t be converting if your attorney can’t help them or has no site content about their case.
Playing in the “vague” pools of searches is inevitably going to lead to lower conversion rates, the goal is to ensure the traffic is still cost effective. Ideally you’d want to pay less for it, if you dabble there at all.