Sorry, but no. There was almost no evidence that the Sassanians were on their way out even with an exhausted empire until after the invaders had taken a good deal of their territory. Even then, Sassanian royal rebels were being housed as far east as the Tang court, and Sassanian Nobility were fighting wars for reconquest in Afghanistan and Central Asia for upwards of a century.
Parthia on the other hand was constantly on the verge of being toppled, and its royal struggles toppled itself several times. It is just the last time one of those happened it led to a resurgence of the Persians under the Sassanians.
This doesn't make conquest likely, but its not impossible.
The Roman Republic isn't more suited to conquering Parthia; both the Republic and Empire would be technically capable.
But an ambitious, wealthy Roman Republican like Caesar or Crassus or Marc Antony had the motivation and will to conquer Parthia. It personally would make him the richest and most powerful person in the Republic, and allow him to rise to the top of the power struggles of the late Republic.
On the other hand, for a Roman Emperor, conquering Parthia isn't desirable. The Emperor is already god among men. He already has the whole Mediterranean basin and everything he wants, so Parthia is an expense to him. The only thing the Emperor would want is to sack the Parthian capital, which multiple Emperors successfully did.
For the Romans, it's the difference between wanting to have everything, versus already having everything and not wanting to lose it.
I disagree with both of you but I can tell that this is going to turn into a long slog with no end in sight. Let's agree to disagree.