For what it's worth ... if there is no title - Dr. or Rev. etc. - or designations - Mr, Mrs, Ms - I use the whole name.
Example : Dear D. Smith - or - Dear David Smith.
So far no complaints.
The rules of address are so complicated - (and there are a few of us left who do remember them !) - that I have opted for the simplest way when the status and/or gender of the person is unknown to me - as above.

You also noted : I'm inclined to change my system to match, although I'm sure ...the older generation ... think .. it ..rather rude.

As one of the older generation I have sighed and accepted it as part of the changing times
.. but ..
what I do find rude is when someone I do not know addresses me by my first name without first asking my permission to do so!
Now, how's that for being a stuffy old phart?
Doesn't it annoy you when you are on the phone and you know the person calling must have your full name in front of him/her and they decide arbitrarily to call you by your first name? Is it so hard for them to ask a woman "Do you prefer Ms, Miss or Mrs?" If Mrs Jones is a Doctor, she will either say so or go with one of the ordinary forms of address.
When you get an answer specifyiing the choice then you say, "Certainly (Ms, Mrs. Miss, Doctor, Reverend, etc). Thank you." and proceed!

Whippersnappers,didn't their mother's teach them any manners or were they born in a barn? mumble mumble mumble .... Not you, dear Drow, those anomymous callers !....and by the way -- WELCOME!