Is your tree in ground or potted. It looks like sun scalding caused by not enough water on a very bright hot day

It's in-ground. I thought I'd been diligent with water - giving it a deep watering every few days for months and making sure the leaves never look too cupped.

What's your location, and how hot is the weather? I live with long hot summers and I would consider a deep watering every few days to be excessive, and my soil is sandy/silty. You should find a way to check the moisture level in the ground, especially down 12" and more...to be sure you're not over watering...which could cause the leaf drop. Most moisture meters won't go down that far, but you can take a pointy metal probe of some kind and push it into the ground. When the ground is wet, the probe will go in easily. You'll be able to tell when you hit dryer ground because the probe will become hard to push into the soil. Or just dig a hole down 12" to see if the soil is wet.

You didn't say how cold it got 3 months ago, but I have two grapefruit trees in containers that handle the occasional 26F night without any problem, although it usually doesn't stay at the level for more than a few hours.

First make sure it's not a watering problem, then worry about citrus greening and greasy spot. Are you in an area with reported HLB, or even the Asian citrus psyllid? You can find photos online of different citrus diseases and nutritional deficiencies...and as I recall citrus greening is pretty specific in it's visual effect on the leaves, and doesn't necessarily affect all the leaves.

Hi, I live on the east coast of central Florida - very sandy soil. We had a dry spell with literally no rain for an entire month, so I was watering it every 3-4 days. It's also a new tree (been planted a little less than a year), which I read typically needs more frequent watering than weekly (what I do with my other citrus tree). I also have a moisture meter I checked and it didn't appear to be overwatered (actually under a bit).

On HLB - best I can tell, it's prominent in Florida, but not sure how to tell about my specific area. I've looked at every online photo I can find but am really struggling to nail it down. I didn't think it was HLB but now the yellowing has significantly worsened and my suspicions are increasing. I will get some additional photos and post them to showcase more of the yellowing.

Your county extension agent can tell you everything you need to know

Unfortunately I tried the county extension agent recently on a separate issue and they weren't helpful (and it took almost 2 months for a reply).

I took some more pictures today focused solely on the yellowing. Each is from a different section of the tree. Does this help anyone with theories on what the cause of this could be? Or possibly to rule out some of the potential culprits?