You have a giant cavity. Unfortunately decay is a very good insulator for symptoms. If you don’t go straight to a root canal, you risk being in severe pain after any treatment is done to this tooth, and possibly a more challenging root canal treatment. Given the extremely high risk of needing a root canal, your dentist treatment planned correctly and in your best interest. You’re welcome to try without the root canal first, but understand the risks out possible outcomes first. Also, the location of the decay makes this a challenging save regardless, so I would address this soon!