Hi Shakey and welcome to the forum! I wouldn't worry too much about the weight of a suitcase on the rack....unless you a using a cabin trunk! Most decent sized cases will obscure the high level brake light. That doesn't bother me too much but I have moved from a large case to using a couple of waterproof soft Ortleib bags instead.
Some people have made brake lights which plug into the loom and then strap to the top of the luggage if it is an issue.
