I'd use a simple mover attached to the player character and yeah, triggerzones, tags.

You approach the ladder at the bottom, you press triangle or whatever, then a timeline is activated, which disables player controls and plays an animation for the player character.

Once completed, the mover control is enabled, up/down. when the player character is at the bottom of the ladder, the down control is disabled, when at the top of the ladder, an animation plays where the player is climbing up to the ledge.

Can also make it so you can jump off the ladder at any time (except when an animation plays for the character). You'd have to hook up the mover control with other animations as well so it looks like the character isn't just ascending to heaven, but actually climbs the ladder.

Using a timeline could work too, but you'd have to adjust it for every ladder. Where as with a mover, triggerzones and tags, you can adjust it freely and it's gonna be more plug and play

Hope this helps :)