Hi Chi,
Latex is among the most durable materials in the industry and maintains it’s properties without softening or sagging longer than other types of foam. There is more about the factors that can affect the durability and useful life of a mattress relative to different people in post #4 here and the posts it links to.
Foam materials like latex don’t tend to feel lumpy no but I would trust your own testing and personal experience more than anything else to tell you how a mattress feels to you. I would also spend more than just a few minutes on any mattress you are seriously considering … hopefully using the testing guidelines in the tutorial post.
Phoenix