I’ve been playing since September, and i’ll give you my perspective of how to use rare candies.
The majority of people will say save them for Legendary and mystic pokemon. I followed this advice as it does make sense to some degree. But it didn’t take long to realise this wasn’t the best advice as the majority of legendary and mystic pokemon are junk.
I’ve learned to prioritise rare candies for pokemon I want to raise quickly but are fairly rare to get, such as Riolu, Gible, Beldum and anything that is good for PvP… especially pokemon such as Registeel, Giratina or any other rare/legendary which you want a second move for.
Perfect IV pokemon are fun to have, but if the pokemon is trash anyway, such as Flygon, then there’s no point using candies on them. The same really applies to all dex fillers.
At this point in the game I use my candies on rare/legendary pokemon I want to use for Great and Ultra league, as it’s a worthwhile investment since you can earn candies back.
Aditional: Despite what I just said I have recently used what I had on Evolving, Maxing and giving a second move to a 96% Shadow Metagross. On the same day I maxed it out I also caught a 100% Shadow Beldum, so am working on that one now.
Use them how you think feels right for you, but weigh up the pros and cons before you do. There is no rush to max out a 100% common pokemon, or evolving just for a dex entry as it will all happen in a relatively quick time anyway.