to me it looks like a teensie issue: you're pushing too many pixels for the hardware.
A teensie is ok for 100-200 leds, as far as i remember, but not much more.
You could test this by using a single long line strip, adding more and more pixels on the go, and see when
it breaks down and is not fluid anymore.
I'd suggest you to by a proper LED controller such as advatek pixlite...
The one controller you suggested https://www.aliexpress.com/item/4000105574296.html
looks only semi-descent: it is cheap, but is it reliable ?.