I took courses in uni for COBOL and you’re right, the language itself isn’t difficult, it’s honestly a lot like writing plain Englisch but making sure your JCL was correct, checking I think it was the spool in order to make sure your jobs were working correctly, reading memory dumps, it was a ride. I love mainframe but it feels like all jobs mainframe ask for 5+ years experience
I had a course in uni that taught us assembler on z/os. My advisor told me most students fail the course on the first try because it was so tough and my Prof for that course said if any of us managed to get at least a B in the course, he’d write us a rec letter for graduate school. That course was the most difficult and most fun I’ve ever had. I learned how to properly use registers to store my values for calculations, I learned how to use subroutines. Earned myself that B and went on to take the follow up course which was COBOL. You’re not crazy, I yearn to go back to doing low level programming, I’m mostly doing ruby for my job but I think my heart never left assembler hahaha