Although Plan 9 has its problems and I am not advocating that it should become mainstream, it has achieved a few things and one of those things is to have demonstrated (by example) that most of the stuff in the OP can be tossed and replaced with simpler things. Plan 9 has seen enough general use (e.g., as the daily environment of a largish group at Bell Labs) over the last 20 years for use to confidently say that much.
There is a program (called IIRC vt100) on Plan 9 that is used to talk to TTYs and serial ports, but it is rarely used, and it is the only part of Plan 9 that incorporates TTY-related concepts. Nor are signals a part of Plan 9, having been replaced by something called notes, which are conceptually different and simpler in concept and implementation.
I remember delving into the documentation of the stty command on Linux a few times in the 1990s. How I wish I could have those hours back (so I could do something more fun or more productive with them).
There are a lot of complexities introduced by the need to handle situations we may never again encounter. The challenge is to make everything simpler, not only in lines of code, but in abstractions.