It is not for everyone. The qualification process is difficult, and harsh. Only about 20% of those that start the process make it through to become an instructor. There was a training company that advertised on the telly, and would take on anyone. They would take the fees upfront, with no refunds when the poor souls dropped out. So many people took their redundancy money, 'invested' it in the training, then later found it was not for them. I trained with BSM. They assessed you first, then offered several ways to pay, although all upfront was the cheapest. If they thought you were not suitable, they'd tell you. Then if you dropped out, you got a refund of any monies not yet spent on training.I looked at joining the ranks a few years back and it was clear it was going to be not only hard work but a doubtful earner for one with no experience of the business, so I decided not to get involved. Any one 'man' band self employed jobs are hard, and as you say, if you dont do the hours it will leave you short of money so high risk.
You start with a theory test. Like the learner one, but with a higher pass mark. No restriction on number of takes, but needing more than two raises doubt about suitability. Once passed, a 2-year clock starts. Fail to qualify in that time, and you wait another two years before you can start again.
Then there's a driving test. Conducted in a similar way to the learner test, but an hour long, includes all manoeuvres, emergency stop on a 40mph road, and motorway if near one. Max 6 minor faults. (Learner has a max of 15) You can take this only 3 times. If you fail a third time, you're out, and need to wait 2 years if wishing to pursue again. Once the driving test is passed, there is an option to start working, as a trainee instructor (Pink triangular badge. Full instructor is a green octagon. Must be displayed in the windscreen when teaching). This is under a recongised training regime, with some lessons supervised, and a training syllabus being followed.
Then comes the teaching test. When I did mine, a senior examiner would role play a learner, describe a scenario, and off you go, picking up and addressing faults. After 30 minutes, stop, different scenario, and off again. You had to control the lesson, and stop it if necessary, and perhaps divert it to address issues if necessary. All part of the examination. so many people failed when they allowed the examiner to approach junctions too fast, or other issues. It plays with your head, knowing he can drive well, but you have to deal with what you see, not what you know. After it is over, you're left for 20 minutes, pondering your performance, while the examiner writes his report. Then he comes back out and gives you the news. Again, fail this 3 times and you're done. Nowadays the test is different. They observe you conducting a lesson. This is why trainee instructors just disappear, they don't tell their students that they are taking their third teaching test, then if they fail, they cannot teach again. So many just disappear, and ignore calls, etc. With decent trainign organisations, like BSM and AA, they have alternative instrcutors lined up to take on students for anyone going for their third test, although they'll have done as much as they can to address all issues.
Once qualified, we get re-assessed, they observe a lesson, and you are rated A, B or fail. If a fail, you get another check very quickly. If that is failed, you get a senior examiner and a last chance. Fail a third time, and they walk away with your licence. (That's the ADI licence, not the driving licence.) Keeps us on our toes.
Then, as Panda Nut says, no guarantee of work. Soon after I started, we went through a period of too many instructors (so many trainee licences) chasing too little work, due to an economic downturn, and lots taking training, but ultimately failing. Covid readjusted that.
Sorry, went on a bit.