I very much wondered the same thing. I've had a couple of occasions where I really wanted a numerical answer to some finance questions, like for instance keeping an IRA vs paying taxes and converting to Roth. And I want it to take into account my tax burden, how much cash I have available, etc. The simplest way for me to do it was python scripts and Beancount + Fava. I don't even try and use it for keeping track of my finances, I use quicken for the simple reason that I know my spouse can use it as well. I didn't see "tax planning" on the features for Ghostfolio, this seems so very basic to me that I couldn't consider using it. I understand why in a way, there are so many situations. Of course if I only have to model my particular situation it's much simpler to just write a simple model in python.