Parboiling potatoes to make them softer in a stew I recently made ciambotta (like an Italian ratatouille) on two separate occasions, first using russet potatoes, and once using red potatoes. In both instances, the potatoes (cut into half-inch pieces) came out too firm for my tastes (and my wife's taste). Both times the potatoes spent about 45 minutes all-told simmering in the Dutch oven before being served, and spent multiple days in the fridge as leftovers.

I thought that maybe if I parboiled the potatoes first I'd get better results. But I also don't want to overcook them and make them mushy. I've seen a lot online about parboiling potatoes before roasting them, but nothing about adding them to a stew.

Is parboiling a reasonable strategy here? How risky is it? I want the potatoes to be softer, but I don't want them to be mushy either.
45 minutes of boiling is more than enough time to soften potatoes regardless of altitude.

Parboiling potatoes before roasting frees up and gelatinizes starches on the outside of the potatoes that then get nice and crunchy when roasted. It will not do anything useful in your stew.

The likely culprit for your potatoes not softening is probably acid.

When potatoes are boiled with acid (lemon juice, vinegar, tomatoes, etc) the acid and heat will cause the pectin in the surface of the potatoes to gel. This is sometimes desirable for things like French fries but it can make them too firm in other cases.

If your recipe has acidic ingredients you can add them near the end of cooking or at least after the potatoes have had time to soften properly.