I’m trying to get my head around how a ground plane would help low angle and reduce nulls.
Theoretically, it should have the opposite effect.
That is a really strange design but if it’s working for you who cares :)

My experience with the V dipoles is that they aren’t immune to polarization shifts. It’s quite possible that you weren’t seeing nulls in the pattern as much as nulls due to polarization changes as the bird rotates in relation to the fixed ground station.
Adding the strange reflector may be providing a fluke bounce back to the V that helps with the nulls.
At these frequencies, the height of the antenna above ground (as long as it’s more than a few wavelengths) should have negligible affect on signal strength as long as the antenna has line of sight to the transmitter.

Did you end up connecting the reflector to the coax shield?
Is it grounded in any other way (clamped to the support)?

The 137MHz noaa is quite powerful and it’s easy to overthink the antenna. I’ve had excellent results from almost anything I’ve used. From a 250’ piece of electric fence (no balun or anything to restrict bandwidth) 5’ off the ground to my 144/440 commercial ham radio antenna.

In my experience, there have only been two factors affecting reception of 137MHz birds.
1) polarization shifts causing sharp nulls during the pass
2) Line of sight being blocked by trees, buildings, other antennas, etc.

The QFH is known to be the best compromise, simply because it’s relatively immune to polarization changes and is still also pretty omnidirectional.

You do NOT really want an antenna with gain in the vertical direction. That’s when the bird is closest so it’s just wasted.

Gain comes from somewhere… it’s not magic.
if you have gain in any given direction it’s at the expense of gain in another direction. Take a balloon and squeeze it into a donut. You’ve just made a vertical dipole. Now push in one side (you will need three hands) you’ve just made a yagi, assuming you didn’t pop the balloon.

My first weather sat receiver was a modified broadcast FM tuner, an oscilloscope for a display with a Polaroid camera to capture the image and a reel to reel tape recorder to keep repeating the pass till I got the scope tweaked for any kind of image. I did use a multi element yagi on that as it was quite deaf so I spent much time on the roof manually tracking the bird.
I’m sure there are school safety regulations now that wouldn’t let it happen! Sure wish I had some of those recordings or Polaroids now!

Have fun, experiment and whether it works or not, try to figure out why.
Christopher