Without trying to sound rude, you're asking the community for wage rates and then telling them it's not right.
500 baht per day is indeed an OK wage here depending on the work, location etc. 7-11 workers work about the hours you're suggesting and earn in the region on 9k baht per month. Earlier you mentioned that nurses earn 50k... a few might, but the vast majority will be on well under that. Those nurses care for dozens of patients. You're asking her to look after 1. The workload will be cushy compared to what would be expected from a Thai employer.
18k would be a VERY fair wage. Live in, with meals, less than that.