So I've finally implemented the specifications set out in the beginning. I've learnt about the 8051 architecture, the Small Device C Compiler (SDCC), the HD44780 LCD controller instruction set, and the I2C protocol.
I'll bear in mind the modern 8051 derivatives for future MCU projects as they are readily available and cheap. The couple of ancient ones I have will be returned to my old artefacts collection. 😊
SDCC is eminently suitable for 8051 MCU projects. I could do everything I wanted in C.
The HD44780 based 16x2 LCD display has limitations but I understand better where it can be used.
I've tangled with the I2C protocol and learnt through mistakes.
I won't be making a production board as this is not a practical gadget; see my remarks in the Details section. It was always a self-assigned learning project.
Now I can reuse that breadboard for the next project!