I can become trained with any Advanced weapon through the Weapon Proficiency feat, if I take it enough times. How can I become an expert with it?
Taking the various Weapon Familiarity feats (like Elven Weapon Familiarity) make Advanced weapons related to an ancestry Martial, but what about a Daikyu for example? How can a Ranger become expert with it?
Fighters naturally are trained in all advanced weapons and become expert in one weapon group (axes/swords/etc.) at level 5 with Fighter Weapon Mastery, master in one weapon group at level 13 with Weapon Legend and expert in all others, then master in all advanced weapons at level 19 with Versatile Legend.
This unusual archetype from the Agents of Edgewatch adventure path has a 16th-level feat Performance Weapon Expert which grants expert proficiency in the advanced weapon chosen as part of taking the 10th-level dedication feat.
Several advanced weapons have an ancestry trait associated with them, allowing them to be wielded as martial weapons by characters with the appropriate weapon familiarity ancestry feat like Dwarven Weapon Familiarity. Humans have a variant of this in Unconventional Weaponry allowing them to do so with any uncommon advanced weapon common in another culture, even those without an ancestry trait.
Fighters have a 6th-level feat Advanced Weapon Training, allowing you to treat any weapon group of advanced weapons as martial weapons. Any character can pick this up with a Fighter Archetype requiring 14 Str, 14 Dex, and 3 class feats for the Fighter Dedication, Basic Maneuver, then Advanced Maneuver as a 12th-level feat or higher.
Beyond those options there are also two archetypes which grant specific weapons or groups a similar benefit:
