If you're going to wait for a Mac where you can change anything about the SSD (soldered or not): forget it. The T2 chip permanently married your motherboard to the SSD even if it were socketed in every new mac.I would have loved to buy a new 2019 16" MBP. Though like many others, I will NOT buy until they stop soldering RAM and SSD. They need to be upgradable. It is a complete joke that for the price Apple is charging you cannot upgrade it.
[Apple can "remarry" them, but nobody else can]
As to CPU and RAM: just buy enough from the getgo, no big deal.