C.R.
Have you tried giving at least 1000 IU's of vitamin D3 daily? That is where I would start. Make sure it's D3 and not just vitamin D. They make a gummy one that you can get at Target and most other drugstores and at health food stores.
We do use Sambucus but only when we are in a crowded place for the day and it can be used at the first sign of sickness too. I never had a problem with soft stool on it. Could have something in it that causes diarrhea. An allergy perhaps.
Also suggest using two awesome products made by Boiron. Coldcalm and Oscillococcinum . Coldcalm is more for cold symptoms and Oscillococcinum is more for flu like symptoms. Both should be started at the first sign of illness. I know that the Oscillococcinum is suppose to be made by duck liver and I agree it sounds like snake oil but I really don't care, it works. I've tested it several times. Both of the products have saved us so many times and I highly recommend them.
C.