>There's a lot of room for someone to create good tutorials and material for ramping up and out of Arduino. Would have helped me a lot, at least.

Good news they do exist (at least if i understood your statement correctly), You just need to search with the keywords "AVR", "atmega328", or whichever version you intend to use instead of "Arduino".

I attempted to make a simple PLC this summer and these resources were a great help :

[book] Make: AVR Programming https://www.amazon.com/AVR-Programming-Learning-Software-Tec...

and this complementary (or introductory) video to the book by the author i believe https://www.youtube.com/watch?v=ERY7d7W-6nA

[Youtube-videos] A playlist of Cornell's ECE 4760 AVR Lectures by the very chill "Bruce Land" (from 2012) : https://www.youtube.com/playlist?list=PLD7F7ED1F3505D8D5

The course website (labs, exercises and readings) : https://people.ece.cornell.edu/land/courses/ece4760/index_20...

Homeworks : https://people.ece.cornell.edu/land/courses/ece4760/homework... Labs : https://people.ece.cornell.edu/land/courses/ece4760/labs/old...

Although the course was centered around an atmega16 or something? i really don't recall sorry but the knowledge i gained was invaluable and easily transferable to a 328p.

Fundamentals of Microcontrollers - Arduino bare-metal breakdown (A playlist of a guy "Mitch Davis" exploring an atmega328 barebone and using tools like AVRdude etc..) https://www.youtube.com/playlist?list=PLNyfXcjhOAwOF-7S-ZoW2...

Atmel Programming Tutorial (by Chris Dahms ) https://www.youtube.com/playlist?list=PLoLaqVexEviMZu55Y4JO6...

Hope this helps.