Hey Vanness - where are you?? I'm in Brisbane

I really feel for you having to leave Matty in care, it is heart renching isn't it. I'm also a full time working mum.
OK - my thoughts are this - once you shift the wake up time sideways that will become Matty's regular wake up time (eventually) as his biorythms adjust. That will mean that if you are waking him up at 5.15am you need to be prepared to be getting up with him at 5.15am EVERY DAY (weekends, holidays etc).
Now that might not be a problem for you because you are used to those early wakings, but I can tell you that would be pretty nasty in my eyes

So I would do everything I could to let DS sleep until the last possible moment before leaving the house. Eg - can daycare give him his breakfast? Will save you time and mean that Matty can sleep a little longer. if I have to leave early I get up, shower, dress, hair, make up and breakfast before waking Fraser, then feeding him, nappy change, dress him and leave. I can achieve his part of the getting ready process in about 30 mins - so in your scenario I would be waking DS closer to 6am than 5am.
OK - now for the practicalities of changing the wake up time - I would work on a gradual adjustment - 5 - 10 minutes earlier wake up every 3 - 5 days (and completely adjusting daytime routines throughout to bedtime) until you get to the desired start time. I know Fraser's wake times fluctuate about an hour so I imagine often you will be having to wake Matty in the mornings - just do what you can to get him to bed 12 hours or thereabouts before the time you are waking him so he is getting sufficient night sleep.
Oh, one thing I would add (completely unrelated

is I have maintained breastfeeding while going back to work. Yes it means pumping and only some jobs can accomodate that but the great thing about it is DS and I have the opportunity to re-connect each night with a really special close time. It's been tiring and hard - but I'm really glad I've maintained the feeding. Anyway - just thought I'd throw that out there.
Good luck!
Jo