Machines trick you into thinking you're progressing. The first time I went from Smith machine squats to free weight squats, I couldn't walk the next day (went from 150kg on the machine to 80kg free weight).
Have you looked into isometric weight training? Heavier weight, lower reps, static holds for longer duration. Typically used in physio for rehabilitation but can also work as exercises on their own, especially when you get older and have joint issues.