Good question. In general, I would ignore anything that uses a generic formula (i.e. any metric on an off-the-shelf watch) and find out what is specific to you.
First, find your aerobic threshold heart rate with a treadmill drift test. (Search this site for lots of info on that.)
Second, keep all of your training below it in intensity. For volume, only do enough volume so that you can do the same workout length day after day. If you can recover in 24 hours, then I’d say your volume is at the right level to not threaten immunity.