In my experience root bound plants are much more rare than over/under watering. What’s your schedule or frequency for watering like? I’d go with that first. Then if after 2-3 days of letting it dry, it doesn’t stand up, you know it’s something else. People grow QP’s from solo cups without getting root bound. Check the thread.
If it is root bound, just transplant, and carefully remove yellowing roots, avoiding any part of the central tap root, it will absorb the rest and heal when it stretches most times it’s not a loss especially in veg. Just keep in mind once you transplant and it recovers, it will use the extra space to go up in size and make new plans about being ready to flower.