The mashgiach of our Vaad haKashrus, who I check with about these things differentiates between different brands, about the frozen veg, there are very few on the approved list. One brand that is, for the most part, is Eden. (not every product, but prob. most vegetables.) Based on that, I would say that for sure, you need a hechsher.
One of the things that are done by the companies is to "blanche" the frozen veg., which is partially cooking them. Obviously that creates more problems, both with basic kashrus, and with some things bishul akum. and it's one of the things they look out for.
I remember, decades ago, when it was discovered that frozen veg. could have problems, and we were told not to use them, so without a reliable hechsher, wouldn't go there.
I have a confession to make: I don’t love cauliflower rice. I think it tastes bland and would much rather eat regular rice with my dishes. The exception is the Tattooed Chef Organic Riced Cauliflower Stir Fry stocked in my freezer. This side dish is incredibly satisfying and bursting with amazing flavor. (I am just as happy eating this rice as I am eating stir-fried rice from a restaurant.) It gives my meals a boost of fiber, vitamins, and minerals, thanks to the cauliflower and other veggies that it contains.