I find that when I have more than one website on the same subject .. I don’t know where to put new content.. they end up almost mirrors of each other, and one ends up for sale..
I do have a casino site.. a poker site that has a casino/bingo/sb area.. a poker forum site.. and a blog or 2 between myself and my partner.. but when I start a new site.. in one of those areas.. it tends to get merged with old sites and 301’s go up.. or it gets sold..
If you’re the one building, updating and so on.. then the new ideas are yours .. in which case.. you’re going to have the same ideas for all of your sites..