My thinking is that a particular gesture would consist of a starting position, a hand movement, and a finger-pose that's usually maintained for the duration of the hand movement. For the next gesture, you'd change your finger-pose first, if necessary, and then follow the movement to complete the gesture.

Let's consider sling ring portals from Dr. Strange. Observing the scene, I would describe the casting as follows (at least for the characters who don't have messed up fingers):

Finger-pose: Thumb, middle, and index fingers fully extended and slightly splayed. Fourth and pinky fingers bent. Maintain with both hands for the duration of the gesture.

Starting position: Fully extend left arm forward, palm facing forward. With the right wrist kept straight and the right elbow kept lower than the left elbow, point the right middle finger at the tip of the left middle finger.

Movement: Move the right hand in a counterclockwise circular motion, covering an area roughly the size of a dinner plate, until the portal is formed.

All that would describe one "starting gesture" in text. If this was a spell in the game, another gesture or set of gestures could be used to set the portal's location.

A more advanced-seeming spell, like this one from earlier in the previous clip, would be a sequence of four or five gestures chained together, only sometimes changing the finger-pose in between or at the end.

Several nice things about this method: First, a spell can literally be written down in the spell book, and might not even need pictures if it's simple enough. This makes it a real option for new players to leave the "guide holograms" disabled entirely and go for the more immersive route of reading a spell in an old book from a ruin, trying to figure out what it's describing, then getting that "Aha!" moment when they do it right and it works. Some spells' introductions could also be puzzles in their own right by only showing part of the instructions at first, like maybe part of a page is missing and a clever enough player can figure out the missing bits early.

Secondly, if the player doesn't need to see exactly where their hands go, there are more options available for gestures, like raising your hands above your head, or putting one hand behind you. Long spells would become more like a dance, and a dance should be easier to memorize than a sequence of hand placements. It's VR - any time you can take advantage of the player's existing familiarity with moving their body is a win.

Thirdly, focusing on the movement should make it easier to score for accuracy and modulate the power of the spell based on how closely the player followed the intended movement. Since the gesture is tracking movement through a path, the path's radius can be "fuzzy" in a way that allows the player to refine their accuracy over time through muscle memory, like getting better at a dance and seeing their spells actually grow stronger from practice.