Its such a wide field! I career changed in my early 40's, to a horticulture based role. To an extent, it depends on where you live - is there a brilliant botanical garden or plant-collectors garden a commutable distance? Is there a great plant nursery near? Or is it more commuter-belt?
Loads of people need someone to cut hedges, shift concrete slabs, do general heavy work. Its unlikely that someone 50 plus would have the physical strength of a 22 year old man, so that type of work is probably not a great fit?
Then theres the weeding, lawn mowing type work - pay usually a bit crap, all seperate customers so an amount if admin etc. Doesnt strike me as a profitable business nor as being that pleasant if you can earn more doing something else and go part tkme or retire earlier.
But a whole bunch of other people "inherit" a garden when they move house. So they want someone to prune, weed, label, and suggest how to make it work for them. So not a real garden redesign, not every day maintenance, not all heavywork, but a reasonable level of knowledge and an ability to spot what the person needs and wants from you.
Then theres NTS, also botanic gardens, that type of thing - generally badly paid but interesting.
I am not a natural designer, nor have I come across any particularly good ones. Generally, people want a jaw-dropping garden year round, with country cottage flowers from March to October in their north facing space, but ultra-low maintenance, all fully grown immediately, with all overlooking windows obscured, and a budget if £1 per square metre. Oh, child and dog friendly too, with red squirrels and hedgehogs. Because thats what theyre paying you for!!
I couldnt be doing with that.
In terms of courses, dont do the rhs level 2. Its pointless. You could sign up for the exams, use past papers and the rhs website and reading lists, but dont pay for a course.
Consider a chainsaw course though, as thats really useful for picking up odds of work from other gardeners.