To elaborate quickly on psylensse's post:
SS cards you should try to get (with lots of patience):
Seraph, as mentioned, gives you +3 to refess, can easily be obtained from seasonal lotto, if you're not being lucky with packs and all.
Cait sith, is in practically every refess prebuilt so far, it works wonders for both lower level rushes (SK, solar kingdom) and big yellows.
Kaldrao (Ex card), 3 LP SS that does 10 damage to every card on the field, usually to trigger your star dragon.
Resource (5*), 3 LP SS that reduces all units' max HP by 10, both can be used in conjunction with burning sun.
Anything with 3 LP, if you're lacking the two mentioned above, which you most likely do.
I can see why you've put the Healing priest as your first (Gowen +1 and Refess +2, wasn't it?), but that's really only a temporary solution.
As psy's mentioned, you WILL need another powder and unwanted charity (I've seen people replace the latter one with general's order, with mixed effects)
Having 3 vet paladins is a waste of space, he already has great survivability, so if you can get at least one more copy you should be fine. (For more info on 2 vs. 3 copies.
Getting a third star dragon is obligatory, as you should know yourself.
Other cards you might want consider getting:
Bastandorra (3*) Lv8 unit, can be used as an end-game unit once your star dragon dies, works wonders IF you have the SP necessary to fuel him. Place the vet pally in front of him and you got yourself a pinpointing and/or mass-slaughtering endgame unit. (Only requires 2 copies as such)
Shadow spirit, low rarity, good damage, decent HP but might scrap itself.
Cards that deprive the opponent of the ability to kill you, like Key to the holy realm, good card for practically every file, Sunbeam Cage etc.
True retear and puppy retear (Both 5*, would not count on getting any until you hit level 100). Puppy can revive one of your units, while playing the role of a tank later on. True retear can mass-revive your field and returns units every turn (except himself) from your graveyard, so it's a must get if you plan on playing big refess in the long run. Again, 3 copies (since you just happened to mention it) is a waste of file space.
Aura of the holy empreror, (4*, grim). Revives a closed unit and if that units level is x or higher (can't remember what levle it was exactly) two cards of the same level will be returned to your file from the graveyard. (To use this on a star dragon you'd have to remove your holy dragon though).
Hand of god (3*) is a staple grim for Refess. Not much to say.
Emerald Carbuncle, low rarity, a pretty standard open (expect a lot of backlashing though) which can protect you from early shadeing.
Other big yellow options include Fistara, Coatl + sun worshiper + mammoth/holy dragon, or Marduk + Oseon + starmander.
As for Shrine Knights (SK), they play very differently. They basically overrun the opponent before they can hit you. SK can beat stuff that Big Yellow can't, and vice versa. Sk return (SK paired with return cards, field control) can beat stuff both other files can't and vice versa again. You can't really compare them, it all depends on the cards you got. (You probably already know about the SK prebuilt) Generally speaking, SK can be a very deadly force, if played right, but require much more strategizing than Big Yellow does. I consider SK to be the more-fun file to play.
I agree with Psy, you should just spend some time watching others, and some time in the chat, it helps wonders (at times).
Packs can **** you over ever so often as they actually help you. A friend of mine just got a BKR in one pack after wasting good money on packs getting absolutely nothing. My best bet would be prebuilts (even though you said you have no interest), hyper rarity and sphere lotto, if they ever appear again. If
you're lucky you can get a bunch of cards in the current treasure battles (set to 100), or at least a decent amount of miku collectors for the point cards.
Tiers are a hard thing to do in the current meta, Teru tried to open a threadhere
. (What happened to Teru anyway?)