While you may be right that the total amount of mercury is not an issue relative to other sources, allergies to thiomersal are a real thing. Here's a page on it: https://dermnetnz.org/topics/thiomersal-allergy/

Personally, I eventually discovered that my terrible problems with wearing contact lenses in the 1990's were caused by the thiomersal used as a preservative in the saline solution I was using. Given my known and tested reaction to having it in my eyes and on my skin, I would be very reluctant to have an injection that contained it.