Hello! Protect and defend are essentially synonymous (the same). I would say that personally, I personally use “defend” more in reference to non-living objects (IE propety, my House, etc). This is by no means a rule. They can practically be used interchangeably IE: “I must protect my son from bullies.” “I must defend my son from bullies.” These are both correct and sound natural. Assess, evaluate and estimate are also synonymous and interchangeable in nearly any situation. The only bit I would make is that “estimate” typically refers specifically to numerical quantities. Also, to assess or evaluate something would infer that you’ve seen it and observed and taken note of it’s featire/appearance/qualities and/or other various possible properties. Estimate tends to refer either to numbers as previous mentioned, or also it refers to making one’s best guess based on prior or common knowledge, averages , appearances, etc. For example, if you wanted to have your car fixed, the mechanic will give an estimate of what he thinks the service will cost based on what he predicts is wrong: it refers to a guess without being exact.