I can't say that there was a certain point at which I made a decision to "go full-time." There were simply points in my life where the next step became clear to me. I won't bore you all with my history, but there's one incident which gives the flavor. I had read a blog article (Zinnia Jones's "That Was Dysphoria?") that got me thinking I might be trans and was seeing a therapist, and during one of the sessions, a kind of voice in my head (I call it my "inner oracle") said "you're going to transition. Just thought you'd like to know." That was the closest thing to a "decision" I had during the process.
I eventually started HRT, at the end of 2015. I'd read that with HRT, there comes a point when people start to notice, and I wild-guessed that that meant I should transition (=go full-time) by the end of 2016, so I put together a time-line of when I'd need to start various steps (name change, talking with HR, etc.) to make sure it would all be done by then. It wasn't so much a matter of "deciding" as of practical planning.
And I didn't "decide" to go for SRS: once I'd transitioned (everywhere), it just seemed obvious that it was the next step for me. I was full-time everywhere (esp. at work) by Jan 1 2017, and that month I got my first appointment at the medical office that everyone I knew said was the right place to get SRS. If it had been up to me, I'd have done it that year, but the medical industry was not (and AFAIK is still not) all that cooperative, so I ended up doing it 6 years later at a different medical practice at a different hospital.
If this sounds a bit funny, I should perhaps explain that my personality has a kind of split -- I sometimes call it "DID lite." My conscious mind handles the day-to-day stuff, but there's an unconscious part of me that seems to have been directing my life, usually without my being aware of it. It feels like a lot of my choices are not done rationally and I'm aware even at the time that my "reasons" are just rationalizations created to keep the people happy who try to tell me how to live my life, but I now think it is that unconscious part -- my "inner oracle" -- that makes the choices, and then gives me the feeling that this or that option "feels right." And go with that because I feel that the "inner oracle" is a lot smarter and wiser than "I" (the conscious "me") am. (Actually, I suspect that the "inner oracle" is the real me, the one that everyone around me wanted to erase, and the conscious "me" is a personality that was constructed during my "hell years" to present me more like what my world wanted to see.)