I know mashed potatoes are ok on purée stage but wondering what other veggies are good to purée.

I personally wouldn't eat mash potatoes because they are very starchy and high carb. You can puree just about any vegetable. Some that work for me are green beans, cauliflower, broccoli, asparagus, spinach, ect..

sweet potatoes would be a better choice over white. I hope this helps.

i have heard you can add unflavored protein powder to the mashed potato to up the protein

Mashed potatoes were forbidden on my plan. Lots of veg works if you cook it to death. But not potatoes or corn.

Potatoes are not a vegetable, sorry. They are high carb and no amount of protein powder is going to change that.
Spinach is a really good one, zucchini/courgette is also good, as are green beans.
Sweetcorn and broccoli caused problems for me

mashed potatoes are listed as an acceptable STARCH on our soft diet.