How do I lessen the appearance of hollow cheeks and sagging jowls without fillers, please?

You could try a skin-tightening treatment like radiofrequency microneedling or laser. How much of an improvement you might get in terms of skin-tightening depends on what sort of shape your skin is in at the moment. Unfortunately nothing except filler is going to replace lost volume in the cheeks. I understand why you might not be keen to try it, but honestly, in the right hands – like those of the practitioners on this site – you could get some lovely, subtle and appropriate results.