My father has advanced dementia, can I prepare a DNR & other advanced care directives on his behalf - he doesn’t have capacity to sign docs.

Your attorney should already know the answer to your question though.....if you have to ask here and then pass on the info to the attorney, you need a new attorney.

Thank you so much for the advice, I’ll give it a thorough read through and pass it on to my attorney for his interpretation.

If you know his wishes. If the family is in agreement with his wishes I doubt you would have a problem.
I suggest a POLST rather than an DNR. The POLST goes into greater detail and is more comprehensive.
The important thing is the document must be kept with him at all times. A copy should be in view in the home or in his room if he is in a facility.
If paramedics do not have the document then as far as they are concerned it does not exist. If they are called they will do what they have been trained to do unless there is a document to tell them otherwise.
I am sure you can print out a POLST for the state of California. Most are typically printed on a Neon color paper because it is easy to see. But it is not required.
A doctor will have to sign the document.

Do the conservatorship appointment documents authorize you to do this?

I should have a read of this, and see how much of it is covered by the terms of your conservatorship. It looks like a useful guide anyway, and it's from your state's website.

https://oag.ca.gov/sites/all/files/agweb/pdfs/consumers/RWJRAEL.pdf

Of course the only difficulty with the website itself is, alas, that it's aimed at people who are planning their own care. Ain't hindsight a wonderful thing? But your father is lucky that his conservator is so conscientious about taking his own stated wishes into account.