Add stock to a sauce pan and bring to a rapid simmer over high heat. Add the ancho, guajillo, chipotle and cascabel chilies, cover with a lid, remove from heat and let the chilies steep until completely soft and pliable, around 20 minutes.
Reserving the stock, carefully remove and discard the seeds and stems from chilies. Add the flesh and skins of chilis to a high-speed blender along with the garlic, vinegar, agave syrup (if using), thyme or oregano, cinnamon, coriander, and smoked paprika. Pour in the reserved stock through a sieve to catch any seeds and blend on high speed until completely smooth. Set mixture aside.
Set Instant Pot to SAUTE mode and preheat 5 minutes. Pat the beef very dry with paper towel.
Add avocado oil and heat the until shimmering. Carefully add the beef and cook, undisturbed, until browned, around 6 minutes. Flip and brown the other side, around 6 minutes. Transfer beef to a bowl and set aside.
Add the onions and season with a pinch of salt. Cook, stirring and lifting any brown bits on the bottom of the pot, until softened, around 4 minutes. Pour in the chili mixture and stir to coat. Return the browned beef to the pot and stir to coat. Bring to a simmer, cover with a lid, set valve to sealing position, and program Instant Pot to manual, high pressure for 60 minutes. Once the cooking time has elapsed, release the pressure manually.
Using two forks, shred the beef and stir to coat in the sauce. Taste for seasoning and adjust with salt and pepper as desired. If necessary, you can reduce the sauce further by simmering using the SAUTE mode, uncovered, until the desired consistency is reached.
Serve the Beef Barbacoa over a bowl of cauliflower rice and garnishes of choice.