Advisor asking me to do review work after my PhD is finished
I think this dilemma can be best understood by separating it into two sub-questions:


To what extent should you be doing reviews after graduation?
Assuming you are doing reviews, does it matter whether they are sub-review requests coming from your former advisor?


For the first question, there are already good answers on this site about number of reviews and about whether reviewing is mandatory.  Both essentially say that reviewing is good community service but never required, and it's better to not review than to give a poor quality or resentful review. Considering this, if this paper came as a direct review request to you from a journal / conference rather than a sub-review request from your former advisor, would you be willing to review it?  If the answer is no (which I think it might be given what you wrote in your question), then the fact that it came from your former advisor is beside the point; you can politely decline just like you would if you had been asked directly.

Now, if you would be willing to review if asked directly, but not when asked as a sub-reviewer by your former advisor, you need to ask yourself why this is, and frame your response accordingly.  For example:


If you are concerned about getting credit for your service and the review request is from a journal, then you can tell your former advisor you're wanting to build relationships with journals and ask them to have the journal transfer the review request (conferences may be unwilling to transfer, depending on how they manage their program committee).
If you're feeling resentful because you want to "leave the nest" and the way that you were asked makes you feel your advisor is still treating you like a student, then you might respond by saying you are very busy in your current position (no doubt true) and suggesting that your former advisor send this to one of their current students instead of you.


Other reasons may have other responses, but these are the main ones that I could think of, and I think may cover the most likely possibilities.