Sadly, the raw Flash chips are soldered to the logic board so they are not user replaceable.
Apple has a custom SSD controller which is part of the T2 chip and the Flash chips require a low level format and a bit of code within them so you can’t use just any Flash chips without prepping them up first.
There are a few outfits which have the skills and needed parts to do this delicate task. But first see what on the used market for the same system with the desired size. It maybe cheaper than altering your system. Don’t forget you’ll loose access to your current stuff so make a backup before you do anything.