I have been on two of the three holidays you mention, and agree with the comments made by jintzky in the previous post, particularly this sentence :- "I would say study brochures very carefully....and think about your priorities." ie food/walking/socialising/sightseeing"
Much depends on how serious a walker you are and what type of holiday you are looking for. I'm a serious walker (eg Iceland earlier this year with a specialist walking company) but also like easier walking to explore new areas as I have a strong dislike of coach travel and being bussed from place to place (which is why I've booked Walking on the Amalfi Coast, and the Loire Valley with JY for 2019).
I'm not a foodie, but as a pescatarian just happy if I get reasonable meals, which I did. I do remember the impressive dining room in the hotel at Alnwick - the wood panelling came from a transatlantic liner.
Would be surprised if large numbers were an issue for walk safety reasons. On both holidays the tour managers were excellent.
Northumberland - I would describe this a serious walking holiday, with the walk along the coast and the crossing to Lindisfarne (Holy Island) using the Pilgrims route very enjoyable in spite of the poor weather.
Lake Garda - easy walking with the exception of one day because of the heat - this area can become very hot. The people I walk with locally would not be impressed with it as a walking holiday, but is better than some in the JY brochure such as Walking the Camino de Santiago which appears basically to be a coach tour. Lake Garda is a lovely area and it was a very good holiday overall, the ferries can be used to visit other areas around the lake.
These are two very different holidays, both were enjoyable.