I am not going to repeat most of what has been very reasonably posted here, and I am starting to get a bit confused by the OP's seeming lack of understanding of the real world.
To comment on supply teaching. I did two years of that and I thought it was a brilliant way to kick-start a career. I did two years on a permanent contract, and then had another child and took off 8 years (part of which was abroad with DH's job).
When I realised I needed to contribute financially to the household, I decided to return to teaching. I started out doing daily and then block supply. It was a brilliant way to compress many years of experience into just two. I could dabble with my teaching persona to see what worked, and being in several schools, I could see how different teachers operated and what was good and bad.
In teaching, you don't get paid for experience beyond the 4 or 5 years it takes to get to M6. If you want to progress beyond that, you have to bring something extra to the school or take on extra responsibility. Everyone has pretty much the same educational background going in.
The difference between a good supply teacher and an ordinary one is whether the school asks for you back, and works out a way to release you from the agency. And if you want to move on to greater things, you get a good reference.
If you do just your hours, count the cost, moan about it, and never go the extra mile, you pretty much get what you deserve.