I never much liked the Arduino Serial implementation. The API is klunky and not having control of the buffer sizes without modifying the platform is meh.

Anyway, it turns out that if you use a Uno4 the Serial implemenation is even more limited. The only write-up I can find is this, but it saved one of my projects: https://www.hackster.io/chuygen/fix-your-arduino-uno-r4-uart-issues-b52e4a