I think everyone's different, and for me personally jumping back into the swing of things is better once you get over the first hurdle and anticipation of going back. For other people doing it slowly helps them.
What you could do is ask for the phased return and do it, then if you feel ready before it's up to be full time tell them/do that. That way you've got the option in place and can do it, but if you feel up to more you can do that too. Where if you say no and end up overwhelmed, it's harder to put in place.
Accommodating appointments should hopefully be an option as well.