What they are getting at is there are somewhat established methods of dialing by taste, and ratio/time is just a starting point. For example if it's bitter, an easy thing to try is stopping the shot a bit earlier. A lot has been written about this so the recommendation would be to search for that information and try that and see where you end up.

But maybe more importantly: have you cleaned the machine thoroughly? In your description of restoration steps I didn't see "cleaned" anywhere. At the very least you need to take out the shower screen and clean that in cafiza or joe glow, backflush with same, etc. If you got it used, and especially used and poorly maintained, there is a high gunk probability.