Does anyone know if Bashing Finish is still bugged?
You'd better picking up Flanking, Improved Two Weapon Fighting or Missile Shield, unless you have a lot of Feats to spare. Even Double Slice helps. You can play and pick up those until they patch it if they didn't yet, since it will take many hours to level to Bashing Finish anyway.
The best classes for Shield Bashing are those which can use some sort of extra modifier to attack and damage (like Smite Evil or Favored Enemy), otherwise the damage is not very good. Maybe Sneak Attack too, or Rage (if it counts regular Shields are melee weapons, because Spiked Shields make no sense in this game, except Ravenna's Oath which is just bugged and acts as basically +10 Shield).
Sadly Magic Vestment: Shield doesn't seem to work. That would have helped a lot. And most of the game you'll be stuck with some Light Shield +3 until you get something better. Not very exciting.
You're way better off doing the standard TWF build. Unless you don't have a friendly Alchemist with the Infusion discovery. In which case: why don't you have a friendly Alchemist with the Infusion discovery?
Enlarged with Ravenna's Oath you would do 2d8+1d6 Bleeding with +8 Shield AC (7 Shield, 1 Shield Focus), +2 attacks for TWF feats (-5 -10) and +1 attack on a crit with Bashing Finish. I don't see that big of a difference. If Magic Vestment Shield actually worked/stacked you would have +10-12 Shield AC instead and theoretically a +9 Weapon in left hand. You will also negate 1st ranged attack which during endgame means possibly negating those 12d6+20 Slay Living seeking (negating all concealment) arrows from main enemies.
In your main hand you can use Holy Flaming Scimitar +4, if you can put Keen and Axiomatic on that and extra +1 somehow you're golden. Or Perfection which is basically +7 Longsword which would add also +2 attack and +2 damage on a shield as well (but that thing is better for someone who can enchant it, like Paladin).
For Ranger we are looking at Bear Scimitar that does some shenanigans on hitting enemy for you and your animal companion and Lead Blades (did not test myself, but who knows? maybe devs would make it so it would work). Might be interesting stuff.
The only problem is starvation for Feats. Once a shield basher, forevah shield basher.
There is also problem with stats which Ranger ignores a lot better though (unless it is still bugged because I heard Ranger feats could turn off if you did not qualify with stats). Lategame +8 Belt would negate that of course but actually lifting off as shield basher early game is problematic. 18 STR is a must then you want 15 DEX and also a casting stat if you're Paladin or Ranger, and at least some CON. And WIS not in the negatives. And honestly some INT to level more than 1 skill also helps. Having 10+ CHA on main and Persuasion is kinda obvious.