Designing your own digital control power supply ain't easy, especially if you don't have much lab gear yet to supply/debug/measure it yet.

Using your USB C source into a "trigger" board to request fixed 20V into a cheapish buck-boost (or even only buck if you don't think you're likely to need >~18V at the moment) module with constant current mode is nice, random example that looks not terrible https://www.amazon.com/Converter-WZ3605E-Programmable-Voltage-Regulator/dp/B09JT1GKH4