How to know what labels are available to reference in user-provided overlays?

I am trying to convert this dts file into a user-provided overlay. I have a work-in-progress, but I'm running into the issue that some labels seem not available to reference with the "&" operator.

Specifically - the uart2 label seems not possible to reference.  Maybe there are more compile errors in my code, but dtc only gives errors one by one...

I tried decompiling the current device tree using

dtc -I fs -O dts /sys/firmware/devicetree/base

There the uart2 label seems missing in that output. I tried whether creating an alias for the path found in the current device tree, but that also didn't help:

uart2 = "/soc/serial@1c28800";

The compile error I got is this:

nanopineo:overlay:% dtc -@ -q -I dts -O dtb -o test.dtbo ./overlay.dts
test.dtbo: ERROR (path_references): Reference to non-existent node or label "uart2"

ERROR: Input tree has errors, aborting (use -f to force output)

I am wondering these things:

- how to know what labels are available to reference?

- the source of the precompiled uart2 overlay seems to reference &uart2, why can't I reference it from a user-provided overlay?

- if the uart2 label is lost in compilation, how can I write a user-provided overlay for uart2?

Link to file I'm trying to convert to an overlay

Link to my work in progress

Device: NanoPi Neo

Distribution: Armbian Bionic