My stock is too sweet
Some vegetables are aromatics for infusing flavor without changing the underlying base, and certain combinations become known for cuisines.  Onion, celery and carrots make mire-poix.  Onions, garlic and tomato make sofrito.

Turnips and parsnips are not those kinds of vegetables, usually.  Parsnips get very sweet (a good thing usually) as do onions and carrots.  You just combined a lot of the ones that get sweet and intensified it by roasting.

If you are looking for a more neutral broth, stay with onions, celery and carrots and simmer, don't roast.