I feel like there is a lot of cultural fear that designers and freelancers in general have when it comes to the idea that they might encounter a client that is so bad for their mental health and business that it will haunt their careers for decades to come.
I don’t think that the idea of treating professional relationships with the same terms and convictions as we treat the personal ones is particularly healthy. A client is not toxic the way a close one can be.
The main reason is that you can always fire a client the same way a client can fire you, so interactions will usually be conducted by both parties in a way to minimize this as much as possible. Clients will usually behave towards you so they don’t have reasons to fire you and you can help by educating them how to do that.
Red flags in this professional context can be misleading; sometimes, the worst professional experiences come from the people that seem the most promising at first.
Trying to avoid people that will be hard to deal with is near impossible because everybody can be hard to deal with, including yourself. To go from hard to deal with right down towards toxic is a stretch.
Yes, there are predators and people with such extreme qualities that they can be a pain most of the times, but don’t jump to the conclusion that someone is toxic because they had a bad day and did something moronic to you.
The job of a client is not to be perfect for you.