Well, if your voice is where you want it to be, then presenting full-time might be an attractive option.
Most people are capable of concealing the effects of HRT on their bodies for varying lengths of time - for many, this is possible indefinitely. Do what you must, with consideration to what you want, with the goal of making your new job the easiest and most enjoyable experience it can be.
In your situation, I went full-time a couple of weeks after getting my job. If you can, it's better to go full-time before your co-workers get familiar with you, but you can't rush yourself if you're just not ready yet.