Sign up for emails from Grand Legacy. They often do flash sales (sometimes they are prepaid non-refundable but not always) and with the amount of time you have until then they are bound to have some and maybe even one that works for your dates. Tropicana and Camelot used to do sales but they are very far and few between, but wouldn’t hurt to be on their email list just in case too.
Fairfield is a fine option, but if you are considering using your points there are much better options, even if they are nearer the convention center (I know you are hesitant to stay that far but maybe you can budget in Uber/Lyft’s as it’s such a quick ride for any nights you are leaving extraordinarily late after park closing).
The next closest Marriot is the Springhill Suites and Residence Inn just a door over on the corner of Harbor and Katella. And also right there is a Sheraton that we used on our SPG points before it was Marriott. I would use points on any of those over Fairfield- with the Springhill or Residence Inn bfast will come with it. Not sure if it’s still the same at the Sheraton but many times we would book it and be able to use points to add club level that came with breakfast and that was a very nice way to get some good bfast and snacks (if we took a midday break late enough for that advantage). But I definitely hear concerns about the general darkness in this area and if I were to go solo, I would stick to coming back at times when most people are to have the advantage of the biggest group of people with me on my walk back. So hopefully something else along Harbor comes through with a sale that works with your budget.