Right time to have second baby?

I would wait until her studies are complete. I chose to have a baby when my son was a little past two. He has a pretty clear understanding that a baby is coming but they will still be similar in age. Not quite the same as PhD but we are starting a business right now and I am seven and a half months pregnant. It's a LOT, running after a toddler and setting up everything for the business and being exhausted and uncomfortable from being pregnant. And that's with a thankfully pretty healthy and easy pregnancy. When baby 2 is here I will be useless to the business for months because I will be juggling two kids.

I thought I wanted a two year gap until I found how it was horrible to try and breastfeed or put to sleep a baby whilst running round after a toddler

We are going for two under two! We know it’ll be a challenge but we want a small gap and I’d kind of rather rip the bandaid than have a toddler for the rest of our youth.

For me it was a lot about our age and looking at the increase in genetic disorders as women age. I am already a high risk pregnancy and I do not want to add geriatric pregnancy on top of that.

I’m still studying and finishing my degree will take longer than we’re willing to wait. For us it was more about how well recovered I felt from our first and how independent and understanding our son is. He will be 3.5 and for us it feels like the best of both worlds: them being close together in age but our oldest also being more patient, independent and understanding.
From a career and studies perspective I personally think that time passes anyways. There are (in our case) no massive benefits if we wait 2-5 years more, so we might as well do it now and figure the rest out as we go.