If you want to go down the DIY path, I have some hats that I am working on, just depends on how much stuff you want - I just had some @Michael Lane hats built for spares and some purposes. I will have some boards left over and will sell them. This is @Michael Lane old tindie store that has most of the information - https://www.tindie.com/products/ranthalion/ml-reef-pi-hat-goby/I have an unused pi4 high speed (can't remember which exact one, I think 8gb?)
I need to see if this $70 leviathan board is all I need. That is attractive. Going to do some digging.
Edit: @Tom Bishop looks like I owe you a beer. With plug and play leviathan costing $70 and the pi4 I already have, it looks like it's absolutely straight forward. I'm going to do more digging later, but seems like the best option.