Should I use my rare candy?

I think you answered your own question too

Just save them till mew or mewtwo. Manhattan has a lot of people per raid saw 40-50 sitting right outside centeral park for a lugia raid. You really don't need good pokes when there are 15+ people per raid.

"Save your candy"

Or something like that. I forget the exact quote that some people took way too seriously

I wouldn't if I were in your shoes. It seems like you're already set. Like you've said, you answered your own question. I probably wouldn't even use Rare Candy on Legendaries as well. I know they're probably the best candidates for these candies and I am excited to get my hands on all of them but I probably won't be using them in battle. It just feels half-baked. I feel more attached to the Pokémons whom I struggled to catch... Walked miles... Been with me from the start. Kinda stupid probably but I don't want to replace my hard-earned attacking Pokémons with a good IV, good stats, strong Pokémon I got in an instant.

Why would you need 191 candies? It is already level 20 right? You need 66 candies to bring it to level 30 (and the 75k dust), unless you have found a way to fatten him up... :-)
Edit: But you probably don't want to keep him a Larvitar... so forget the above.

My suggestion would be to keep him as he is. If you have all ttars with the movesets you want then no need to spend anything on him. I would keep him because they may change or replace movesets in the future and if so you already have a high IV to evolve if you need to.

Since it doesn't rain T-tar candies on you, why would you want to use that many candies when you can easily hit up a T-tar raid and:
A) get exactly the MS you want, or
B) use TM
Sure, if your larvitar was 100%, then go for it for bragging rights, but really the difference between any 80%'er (with 15 attack) and your 93% is minimal.

I live in suburbs, and even with limited playing have been able to add 6 T-Tars to the one I hatched (at 98%) and walked "back in the old days" - personally I'm waiting until the raid bosses rotate to figure out which Ttar to power, which to TM etc. The TM drop rate for legendaries is pretty good....

Catch two T-Rex's in raids instead and use the candies to power them up.

Watch for Tyranitar raids. Also, the event has been extended, so wait and see if you hatch another Larvitar before spending your rare candy. There are worse things you could waste your rare candy on.