Avoid the Tallahassee Marriott, and asking for help on how to fight this

LD went on a business trip to FL with colleagues a while ago and stayed at the local Marriott. At first, six nights were booked. After the first night, LD developed a lot of very itchy red spots/papules, mainly on the legs, ankles, abdomen, and other areas. She also saw very small bugs in the room herself, but they moved very fast and she did not get a photo. The next day she went to the front desk to report the issue, and the hotel moved her to another room. But after the room change, the situation was not resolved, and she also saw bugs in the second room, while the red rash on her body continued to increase and stayed very itchy. Because it was really uncomfortable and she is a poor student, she did an online consultation through the WeChat mini program. The doctor looked at the photos and judged it to be papular urticaria, consistent with insect bites. Those are all the facts, but there is no substantial evidence such as photos of the bugs. The original plan was to stay a total of 6 nights, but after the room change the situation still did not improve, so on the third day she decided to check out early. Her colleague also had similar symptoms. At that time she went to ask to cancel the remaining three nights, but the manager argued with her for a long time. According to LD, the attitude was fairly aggressive, and only in the end did they agree not to charge for the remaining three nights. In the end she actually stayed three nights and paid for three nights. Later she switched to Hyatt House, and only then did it stop getting worse. About 20 days later, LD was still very itchy, with scratch marks all over from scratching, which looked pretty scary. The more she thought about it, the angrier she got, so she organized what had happened and complained to Marriott, submitting photos of the rash, screenshots of the online doctor consultation, and an English translation. Then Marriott transferred the case back to the hotel to handle, which seems to be Marriott's usual practice. Then the hotel manager called LD. I was not there for the call, but LD said the experience was very bad. The manager kept fixating on one point: "Do you have photos of the bugs?" LD explained that she had photos of her body, that she had reported it to the front desk during the stay, that the hotel had changed her room because of it, and that the doctor also thought the skin reaction was consistent with insect bites, but the manager kept harping on "without a photo of the bug itself, there is no evidence." The other party also had a very forceful tone on the phone, constantly questioning that she had no "direct evidence," which made her extremely nervous. P2's English itself is not especially good, and with the emotional pressure, in the end, just to end the call, she said that refunding the three nights already stayed would be enough. This time the manager was very quick to agree and directly agreed to refund all of the more than 400 dollars for the three nights. The question is, if they really were fleas or very small bugs that move around, how is an ordinary guest supposed to get a photo? Personally, I think the evidence chain of "a large number of itchy rashes appearing right after check-in -> the problem still existed after changing rooms -> the online doctor considered it consistent with insect bites" is already fairly complete. And now, almost a month later, her body still has not fully recovered and it still itches. But based on my experience browsing Mudan, in this kind of situation a refund alone seems a bit insufficient. Should we keep fighting? Mainly because of the manager's attitude and because with this kind of bed bug-type problem, I feel some goodwill compensation should at least be expected. So what I am conflicted about now is: after the full refund for the three nights, should we stop here or keep fighting? If we keep fighting, can we have Marriott handle it directly and request no further communication with this manager? Also, the manager's attitude makes me a bit worried about retaliation. In the future I may still need to go there often for business trips to do experiments, so I am a little concerned. Has anyone encountered similar Marriott DP? In general, how far does Marriott usually take this kind of complaint?