Love the work. Some small things I've noticed with the hud:

• The hud seems to "jitter" when rotating due to the decimals disappearing when the angle is a whole number. Is it possible to pad the value with a trailing zero to minimise this?
• "vV" and "v" seem arbitrary unless you understand that they're "Vertical Velocity" and "(Horizontal) Velocity" respectively. Maybe "Climb Rate" and "Airspeed" instead?
• Perhaps a label on the rocket cooldown? It's currently just an unexplained countdown

Possibly some less-small things related to the physics:

• Opening the glider at high velocity (only tested by freefalling so far) can sometimes cause instant death
• If the player is killed by overspeed (ie freefalling then opens the glider as above) the player is sometimes also damaged when they respawn
• The glider can still climb higher than its initial launch altitude without using rockets after barely falling off a high enough cliff (ie without jumping, running or even walking.) Maybe some sort of climb-to-drag ratio to minimise this? Or just chalk it up to a good pilot being able to find lift? idk

Many thanks for the valuable feedback.

• The hud seems to "jitter" when rotating due to the decimals disappearing when the angle is a whole number. Is it possible to pad the value with a trailing zero to minimise this?

Yes, simple enough to do. I actually had it without decimals and only added them for debugging. Minetest isn't designed to be a flightsim and the HUD is just to help out a little. I don't think players need decimals in the heading indicator. A compass in the hotbar should be just fine.

• "vV" and "v" seem arbitrary unless you understand that they're "Vertical Velocity" and "(Horizontal) Velocity" respectively. Maybe "Climb Rate" and "Airspeed" instead?

Yes, kindof. Both values are quite arbitrary but do help pilots understand why their glider is behaving the way it does. (v is actually speed factor multiplied to all directions). Your suggested labels are correct. I didn't use them because the first complaint from players is: "Ugh, the HUD is covering my view". Also translations in mind, I tried to go for some ISO standard short labels. Players with small screens quickly have overflows.
Possibly a solution would be to add a chatcommand to toggle HUD and provide options on verbosity. (Originally I had vV only as +, - or =)

• Perhaps a label on the rocket cooldown? It's currently just an unexplained countdown

Yeah, I was thinking of a coloured horizontal bar and that players would notice soon enough what it meant. I'll consider it again.

Possibly some less-small things related to the physics:

Disclaimer: I didn't change anything on the physics.

• Opening the glider at high velocity (only tested by freefalling so far) can sometimes cause instant death

Seems reasonable to me. Something similar would happen if one found a way to do that IRL :D This mod is not the [hangglider] mod which is designed to break falls while this mod is a delta glider (I really want to rename this mod to that). Delta gliders are not safety devices, rather lightweight fixed wing aircraft.

• If the player is killed by overspeed (ie freefalling then opens the glider as above) the player is sometimes also damaged when they respawn

That isn't unique to this mod. It happens on occasion with various mods. I'll call it an engine issue.

• The glider can still climb higher than its initial launch altitude without using rockets after barely falling off a high enough cliff (ie without jumping, running or even walking.) Maybe some sort of climb-to-drag ratio to minimise this? Or just chalk it up to a good pilot being able to find lift? idk

I am unable to reproduce that (starting without moving). Maybe my hardware is too sluggish and old. With a running start, I can rise higher and I did consider that a good thing. It keeps the gap between [hangglider] and [(delta)glider] wide enough for servers to consider using both. Also it motivates players to use this glider at all, it's hard enough to even land safely. This mod is quite dangerous as it is, I consider it a good feature that one can actually gain altitude if one is skilled enough.

Looking at heading more closely I noticed that flying in counterclockwise circles it is accurate. However flying in clockwise circles it becomes negative!

I'm torn between dropping heading information and fixing it. It's the least important of
all the HUD info and was added more as a gimmick.