Hi. So the bottom line is that you go full time when you feel ready for it... and what you mean by full time. For me full time meant legally changing my name (and identification papers, licence, bank accounts, etc), telling everybody and from that point on presenting as female. It really is a deep end approach, perhaps not sink or swim but you need to be sure you can paddle a bit at least. You're right, you learn a lot and you learn it quickly since you are essentially tipping yourself into lived female experience 24/7. While you could have days where you say, "meh, don't feel like the effort of making myself up today (or whatever)" that might not be a viable option. Being part time is easier from that point of view, you can be whatever mode you want/need to be.
I know some transwomen who have gone full time even before taking HRT or having electro, others wait a year or more on HRT before moving forward. Each to their own! A lot of it has to do with the need to move forward and how confident you feel with your appearance and presentation as a woman. When I tried to transition in the early 1990s (at around your age) I never felt overly confident, even though I looked quite feminine after two years on HRT - and so I pulled the plug on that attempt. This time I was on HRT for 9 months before I decided to go full time - during that nine months I tried to get rid of as much of my beard as possible and went out in a variety of settings in girl mode to build my confidence. Once I knew I was ready there was no holding me back.