How to list a list of PhDs

This is what I would do: the comma to separate the person's name from the post-nominal title, the semicolon to clearly separate the list of names, and the Oxford comma to indicate who is the penultimate person in the list.

John Doe, Ph.D; Bill Doe, Ph.D; Jane Doe, Ph.D., and John Smith, Ph.D.

semicolon

Semicolons can also be used in place of commas to separate the items in a list, particularly when the elements of that list contain commas. (Wikipedia)

Doxical Style Guide

1.138 Punctuation
Post-nominals are enclosed with commas, as in “Joanna Klim, FAusIMM CP(Min), is...”.
Do not use periods or spaces in individual abbreviations of honours, degrees, and fellowships. Each category of post-nominal is separated with a comma. Individual qualifications within a category are separated with spaces. Do not enclose the name of the institute with parentheses—they are often used to show a discipline or other qualifiers, for example BE(Mech) and BSc(Hons).