The true Cloak of Invisibility may be using the thestral blood for permanence as well. Most cloaks of invisibility fade and weaken with time until they are useless; until Harry discovered a secret use, the primary difference of Harry's cloak was it's ability to last for so long.

IIRC, both canon & MoR flat out says that the Cloak is more than a merely unfading ordinary invisibility cloak, but grants hiddenness and protection on a deeper level than them. It differs in both quality and quantity, you might say.

I would guess that most people don't have the means or desire to experiment with thestral blood.

One might say the same thing of using or researching dragon blood. And yet...

Lastly, Felix Felicis is a super finicky potion. It is toxic in large quantities, difficult to brew, and dangerous to get wrong.

Everything is toxic in large quantities, and the difficulties & danger of preparation apply to, well, the preparation, not the end product. (This actually makes it an even better potion to make permanent with thestral blood.)