My experience is similar to yours as far as the 3x7 drivetrain and the Shimano Tourney and the daily commuting. I replace rear derailleurs every so often because a pulley wears out or a spring breaks. I also change my cables about once a year and have to re-adjust the derailleurs. I'm no expert. My technique is to put the Park video on my laptop next to the bike and literally go one step at a time with lots of pausing. This has worked well for me.