I’m no expert, but I like to workout in cycles. When I’m lifting heavy and start to plateau, I will tend to switch to a high rep lower weight program for 2-4 weeks. Then start back on heavy lifting, starting a bit lower and working my way back up.
It works for me, but everyone is different. Good luck in your journey!
Edit: when I’m feeling sick, I’ll take really light days. Stretch only, or do some very light weights. I try not to push it while sick and prefer to just rest.