Is it possible to enable a respondent to bypass authentication?

How about putting the authenticator in a branch. Ask them if they have their id or not before branching.

I would only want respondents who are using the anonymous link to go through that path in the survey flow. Otherwise, if they are using a personalized link, I want the authentication to pre-fill and go straight to the survey with their data pre-populated. Is that possible (i.e. branching or different survey flows based on using the anonymous vs. personalized link)?

Nevermind. I have a working solution using multiple branches. Essentially, I created 2 branches (Branch 1 for personalized links, and Branch 2 for anonymous link) after my initial Set Embedded Data in the flow:
1) If ExternalDataReference is Not Empty then authenticate w/ pre-filled fields and set embedded data
2) If ExternalDataReference is Empty then ask a question if they have their ID; If yes then display another authenticator w/o pre-filled values (because we don't have them) and set embedded data upon authentication; If No then take them straight to the survey

You could also skip the first authenticator entirely. Add display logic to the "Do you have your ID ?" question to only show it in case of an anonymous link. Something like if Q_CHL is empty.