Is it considered forbidden to lack feelings of love towards one's parents, and to suffice with honoring them out of obedience to Allah's command, knowing that Allah does not hold a servant accountable for what is in their heart?

Filial piety is among the most emphasized obligations and greatest rights. As long as you exert your utmost effort in it, you should praise Allah. You will not be held accountable for an inclination of the heart over which you have no control, for Allah does not burden a soul beyond its capacity. The Prophet, peace and blessings be upon him, used to divide fairly and say: "O Allah, this is my division in what I possess, so do not blame me for what You possess and I do not possess" meaning the heart.