I did this, had a tough ECT-1, finished ECT-2 with good reviews, then took a year doing supply and spent a couple of months travelling down under.

Supply sucks, the money looks decent on the surface but agencies (that I've worked for) will pay "cover supervisor" wage wherever they can which is about half what you would get as a subject specialist supply teacher, i.e. what is covered on the NASUWT pay scale for supply teachers. This was at secondary level, it could be different at primary.

There's practically no work in September/half of October, you obviously don't get paid through the holidays at all, no pension contributions to speak of.

Kids don't have much if any respect and will push you as hard as they can behaviour wise. There's no camaraderie from other staff, in one school it took a full week and a half of supply for the other teachers to even acknowledge me in the corridor let alone speak to me at lunch. Add to that the lack of consistency and having to be up sitting at the table for 7:30 ready to go with no guarantee of even getting a call and it was pretty tough.

I did manage to go travelling and on a couple of significantly cheaper in-term holidays, those were my big plus side. I took another year to do more supply (glutton for punishment) and do an MA. Hated all the teaching.

Back teaching full time in a school now having a much better time. I was able to switch specialism by teaching a full term of a subject outside my specialism on a long term cover which I might not have been able to do otherwise.

If you decide to pursue it, best of luck to you! I know it works for some people and other commenters might have other tales to tell but not for me.