There are many varieties of squash and root vegetables, some of which aren’t specifically bred for sweetness.
But how the vegetables are grown and stored play a rather significant role in sweetness.
For instance, if you leave many root vegetables in the ground over the winter, they’ll increase their sugar, which you specifically don’t want. But grocery stores might keep carrots and such refrigerated, which has the same effect.
Sweet potatoes are the opposite— they’re ‘cured’ with heat to improve sweetness and storage life.
Some varieties of squash are sweet when really young (the ‘summer squashes’ like zucchini and others picked when small), while others (the ‘winter squashes’ like pumpkins) are let to fully mature to get their full sweetness.
So my suggestion would be to see if there’s any sort of farmers market near you, and tell them what you’re looking for. They might have some immature squashes that they had to pick early because they were damaged, or be willing to sell you uncured sweet potatoes, etc.
They could also tell you which varieties are less sweet. I know white carrots tend to be less sweet than orange, but I can only get them individually at the farm stands. In the stores, I have to buy a bag of ‘multicolored’ carrots, some of which are sweeter than the orange ones.