In your gear, a very good player can be doing 20s. That’s not to say 16s/17s are bad, but you’re definitely capable gear-wise of pugging up into the 20 range if you practice more. And yeah, it does come down to practice/knowledge and getting comfortable.
16s/17s I was doing pre-nerfs in around 434 gear. I still haven’t pushed 20s on my priest this season (I kinda raid logged starting like 3 weeks in lol), but 18s I do with friends when they need help feel pretty comfy to me right now.
Really with Disc it’s about learning the biggest healing checks and planning for them. A lot of people play the spec and stress about their rotation, but you really don’t have to. Radiance covers a LOT of the standard level of damage in a dungeon. And you can keep renew running on people taking minor damage. The biggest learning curve is figuring out when Atonement will cover for you, and when you’ll need to burn CDs. If you don’t start to feel that out, and sometimes let people sit at 70% health rather than immediately topping them off, it can just feel like constant stress. But that really requires practice.
You mostly need to be prepared for the worst of it. IE: Going into BH first boss you need to know to pre-rapture. Fighting the elephant or spear boss in NL you need to have your Schism on the target and a TE enhanced HD Dark Reprimand ready for the stomp/spear breaks. Etc.
Generally, you only really need worry about Twilight Equilibrium for your highest damage spells. You can put it on PTW before you spread it (and spread a larger dot) as well, but it’s not make or break like missing all your bonuses on a giant penance. Managing TE well 100% of the time is very much a high level optimization thing, but if you’re not pushing 24+ keys or something I wouldn’t worry too much about it. In fact, it might actually be hurting you if you’re casting Smites when damage is happening just to make sure you have light side TE or something.
Standard build disc priest you’ll want to be using Pain Suppression and PW:S on targets that take the strong ST dots (there’s a lot of these in Season 2). But you can also use it pretty liberally when something feels like it’s going to be difficult considering Protector of the Frail gives us 2 charges now and reduces the CD a lot.
Also don’t underestimate rapture. It’s actually a very strong cooldown with Aegis of Wrath/Exaltation and it has a very short CD. Remember that when a target already has atonement, your shields from rapture are going to get mastery bonus. So it can be worth it to Renew the party or Radiance → Rapture blanket if you know something big is coming. For example, in high level Underrot, I’ll renew everyone then rapture blanket just before tantrum and have to do way less during it. Also you can PW:S the same target repeatedly during rapture if you need to, so it’s not just an AoE shield blanket. It’s also very strong ST healing when you need it to be.