I learn two things the hard way with my Elternzeit.
One: If you take 2 years from the start, you can unilaterally choose to add the consecutive third year. If you only take 1 year, your employer has to OK any extension.
Two: When they say you can work 30 hours on Elterngeld plus, they mean 30 hours at the same rate of pay that you had, assuming you're just going to reduce your previous hours in the same job. If you do something else (I freelanced from home instead), then they don't go by hours but by the money you earn. At the start of my Elternzeit, I was told to guess what I thought I might earn as a freelancer during my Elternzeit. Never mind I had no way of knowing what having a baby would do to my ability to work, or how many freelancing gigs I would get over that period. They based the Elterngeld on that guess, instead of just using my known previous income and telling me what I could earn and what I would get when I did. Had they told me from the beginning what the formula was by which they calculated the payments, I could have limited my income accordingly. I am currently PAYING BACK €2000 of Elterngeld because it was only after I had finished my Elternzeit that my actual earnings were calculated against what they had paid and got a retroactive bill. I would strongly recommend sitting down (if you're good at math...I'm not so my tax consultant did this for me) and making a table of "earn x, get y" so you can balance how much you want to work with how much you ask for and prevent your total income from exceeding 2/3 of what you earned before.