Umbrella sounds like high overhead, but the higher the limit, the less the cost-- in a manner of speaking. This is because generally, the risks are lower at higher levels. In our world, a mystery shopper is more likely to incur a fender bender or even an injury incident than to be responsible for a catastrophic loss or a lawsuit stemming from an allegation that the mystery shopper's words and/or actions harmed a person associated with a shopper location or the entire location's property or business. Perhaps MF Johnston, Niner, or an actuary can explain this so that it becomes clear.
I would say, be as covered as you can afford to be. Life happens. Be prepared to let the insurance make up monetarily for some booboos, even if you know that you live and drive safely.
My garden in England is full of eating-out places, for heat waves, warm September evenings, or lunch on a chilly Christmas morning. (Mary Quant)