Tooth sensitive but no holes or blackness. X Ray shows shadow (decay)of 2nd premolar nearing upto nerve. Filling suggested but risk of RCT. Advice pls?

Follow. Follow your Dentist's advice. Just because you can't see tooth damage w naked eye doesn't mean there isn't one. Imaging revealed deep cavity. Infected tooth material must be removed to prevent abscess. If root canal treatment is required have it done. The alternative is to wait and be exposed to serious complications.
Have it treated. If you do nothing the decay will progress, the pain will intensify, and you will get an abscess. At that point they may still be able to save the tooth with a root canal, or it may need to be extracted. If you have the tooth treated now (before an abscess develops) you may still need a root canal, but your chances of saving the tooth is much greater. Get the filling and root canal done now. .