Pleading the 5th is tricky.
Invariably, once you start to answer their questions, you can effectively wave your right to plead the 5th at a later time.
IOW you can’t necessarily pick and choose the times you do or don’t want to answer their questions (plead the 5th).
The problem otherwise is to be consistent and succinct in your responses (to avoid a perjury trap), and give consistent non-responsive answers (”I do not recall”) but again avoiding creating your own perjury trap.
Yes you can pick and choose. It isn’t smart, but quite literally;
“You have the right to remain silent. Anything you say can be used against you in court. You have the right to talk to a lawyer for advice before we ask you any questions. You have the right to have a lawyer with you during questioning. If you cannot afford a lawyer, one will be appointed for you before any questioning if you wish. *****If you decide to answer questions now without a lawyer present, you have the right to stop answering at any time.”*****
Taking the 5th is simple as long as you continue to assert the 5th.