Thanks for that! I always forget about the Waveblade because it's not listed as part of the monk weapon group over on AoN.
We had discussed Modified Weapon Prof and the Weapon Adept feats from the source too but weren't totally sure how they'd apply. But after further consideration we realized that EitR doesn't actually remove the weapon categories from the game. Weapons can still be Simple, Martial, or Exotic, so a Fighter would still need to spend a feat to gain access to exotic weapons, they'd just gain a whole weapon group instead of only 1 weapon.
So making a modified a Rapier would make it exotic, but still a part of the Light Blades group, so someone with Martial prof wouldn't be able to use it proficiently without taking Weapon Prof with Light Blades, Modified Weapon Prof, or Weapon Adept. Likewise someone with Weapon Prof in Flails wouldn't treat a modified Spiked Chain as something they were proficient with unless they also had the appropriate Modified Weapon Prof or Weapon Adept feat to match it. Tho to keep pace with EitR Modified Weapon Prof would be for a whole group not just a single weapon.
As for our Modified Rapier, the character wanting the mod is a rogue/monk so they have proficiency with the Rapier itself from Rogue, but also with all Monk weapons from Monk. So modifying Versatile Design on the Rapier would change the weapon's category to Exotic, but also add it to the Monk Group so the character wouldn't need more feats to use it. :)