Muddy, I'm so sorry to hear that. We've been in a cancer life in a similar time frame. I have been on a clinical trial for pembrolizumab. It was put in through an IV, took about an hour in total (including the usual faff). I felt tired for a day or 2 afterwards, but was OK.
For me I did get a rare side effect so can no longer have pembro and had to stop the trial.

Do you know what other drug is going to be put with the immunotherapy? Do you know what immunotherapy is on the table?