As a practitioner who has experience with the slow erosion of the physical, I would have to compare it with mithradytism taking small amounts over time will build tolerance. The death you experience isn’t physical it’s spiritual. It is then possible to die multiple times spiritually. Each death will unlock more dormant abilities sleeping within. I once believed as you did I was terrified of the death essence because of those effects. The death essence is a living thing it kills with precision it adapts and molds to the practitioner. First it opens up the minds eye wider. Then it creeps up the spine and is absorbed into the bones loosening the joints.
If you are truly meant for necromancy the death essence is your best friend. As it approaches the heart it will do two things incense the heart rate and form itself around the arteries and over the heart in a dense coating more like a wire coating. As long as you ask your spiritual team for healing they will give you proper healing rituals to stop or reverse the necrotic damage. Take a few days rest and you’ll be ready for another death working and the process repeats
Remember to progress slowly only by going too fast would you experience
Panic/heart attack
Stomach pain
Joint pain
Dulling of nerves and other nasty side effects