I’m glad you are enjoying making herbal medicine so much, and sometimes the challenges are the best teaching moments hey?
In this case yes, you could add glycerine. There are just a couple of considerations.
1. How strong was your alcohol in this tincture? E.g. If it was 50% you could add enough glycerine to bring it down to 25% in strength and still have a fairly good preservative value. Glycerine also has it’s own preservative value as well, so it will likely still keep for a few years if stored properly.
2. If you add glycerine then this lowers the concentration of the milk thistle. E.g. You are already at a 1:5, let’s say it’s 100 mL and 20g of Milk Thistle and you add 20 mL of glycerine, it now becomes a 1:6 and more will need to be consumed to have the same medicinal benefit.
I hope this helps clarify a little bit!