Not really surprising. That last part "tard" comes from the Latin tardus which basically means slow or late. The "re" particle originally just had a meaning of repetition, in this case. So being late (not for the first time? ) So changing the first part of the word wouldn't change the meaning of it so easily. Maybe technically something like "intard" could make sense in that way (similarly as what you find in words like indomitable, impossible, ineffective etc), but it doesn't really exist. Or if we want to make it more meme-like, tarden't?

Protarded actually exists as a slang with completely different meaning, but that's out of scope

Overall useless information for most, but anyway