If I understand the RAW on climbing correctly, climbing up a cliff means making a Strength (Athletics) check every round and moving half your movement speed. Which means a character with 30' movement speed would need 7 checks for a 100' cliff, and 28 checks for a 400' cliff. Not sure if you can "dash" and move twice per round, and whether that would need one or two rolls. Anyway, that is a lot of rolls to do. It takes forever and even at low difficulty somebody sooner or later rolls a 1 and falls. While that might not be deadly if the character has a climber's kit, they would need to do even more rolls, taking even more time.
In the adventure I am playing, depending on the decisions the players make, they might end up at the foot of a cliff (adventure says it's 400', but I could change that). I would like the players to be able to climb up if they want. I would like the climb to be some sort of challenge, but I don't want it to take forever, nor be potentially deadly. How can I design an interesting climbing challenge, without completely breaking the written rules?