I reach for a similar pattern a lot with postgres as I'm building up a system. Start with a think about the fields I know I want, and create the tables with them, and then store all the metadata I have lying around in a json column, then in 2 months when I realize what fields I actually need populate them from json, and then make my API keep them up to date, or make a view, or what ever.

I've found it really helpful to avoid the growing pains that come with "just shove it all in mongo", or "just put it on the file system", but not much cost.