My brother's husband is a personal trainer. Not sure how relevant his experience would be to you as he doesn't work in the same specialism but I can give you a few more general bits I've picked up over the years.
First thing, DBIL would say if you've considered it a few times it's probably a sign that it's something that really speaks to you. He always says that coaching and people skills that are more important than knowing 100s of exercises inside-out. (That said, a good working knowledge of anatomy and physiology will undoubtedly help, but you can pick that up as you go rather than needing to know it right now.) It's one of those industries where relatability is really important. You need to be good with people one-to-one (or in a group setting, if that's what you're keen on).
It's a really positive, encouraging world. He's often remarked that on his course there were people from late teens all the way through to late 60s. It certainly wasn't all gym-buffed 20-somethings; in fact the majority were career-changers in their 30s and 40s. Everyone was extremely positive and supportive all the way through. He'd come from a corporate background that was backstabby and he really noticed this difference. He made some good friends on the course.
The courses are quite expensive. I think he paid about six grand to do his, and that got him a level 4 qualification with some add-on bits for extra specialisms. He also does two or three 'skill-up' courses a year to keep his knowledge and skillset current. He would say the best training providers are YMCAfit and Premier. Other training providers are available though.
Downsides: the qualification is just the beginning and it can be competitive.
(May or may not be a negative, depends how you see it) DBIL certainly uses social media a lot. He's one of those tall, glossy, triangular men who make Chris Hemsworth look like Johnny Vegas, so he's kind of designed for Instagram. (I love him to bits, he's a lovely guy, but he is ludicrous looking.) He sees it as marketing and about a third of his clients come to him because they've seen him demonstrating a split squat or whatever on a video online.
The hours can be weird. PTs can end up doing long days of split shifts, with client hours concentrated pre-9am and post 6pm, plus weekends.
Some areas of the country are a bit overrun with minimally qualified PTs all competing with each other at the entry end of the market. It can be a bit tricky to stand out. Getting that first job or placement out of the stocks can be tough, and with some of the chain gyms the PTs actually have to pay a kind of 'rent' to have access to their gym floor and members. So it's not a route to riches, at least not straight away. DBIL earns solidly now but he's not on megabucks, and he moved away from working at the chain gyms to get a bit more control over his earnings.
What else can I tell you... it's quite seasonal. Busy in January, leading up to wedding season, and summer holidays. He isn't busy in early spring, autumn or December. This can make financial planning a bit tricky.
Clients can be flakey, cancelling at the last minute or deciding they can't be bothered to train. DBIL has contracting conversations with people in advance now, and only sells his time in session blocks.