I’ll recommend the way I read and calculate the touchdown Vspeed.
The 6Hz polling is fine.
You already read the VS, and it’s x02C8 - the singed offset as 256 x meters/second - so for the appropriate feet/minute the conversion goes like 603.28084/256.
Now, at the same time we can also read x030C - it works the same way as x02C8 (so it’s the vertical speed), however it is not updated while there’s the SImOnGround flag set (that is x0366).
Works very precisely in MSFS, and I rely on that with every FSTD/instructor station I design.