Would it be better to do completely level 100 units in the first year or do some level 200 units in session 2 that you have met the prerequisites to in session 1.
Definitely do some 200 level units in session 2 - because that is likely to be the bulk of your course requirements, in terms of credit points. You don't want to do what I did and stay back a extra session, just to make up credit points
I'd also recommend that you look at previous years unit guides to have a brief idea of when assignments are generally due so you don't have a lot of work due at the same time.
If you've completed the compulsory 100 level subjects then I see no reason to delay starting 200 level subjects.
For example; I found ACCG250 easier then some 100 level courses.