New AST questions

[Anarism]

Hey guys.
First one would be about Royal Road. 3 DPS cards and Bole all seem too good to burn them in 4-mans, so I assume I generally don't use it outside of AoE buff?
What about 8-mans? Do I ever use Balance, Arrow or Spear on single target?
Are duration and potency buffs ever worth it?

There's never a reason to go for duration/potency on purpose but there are situations where it's more useful to Royal Road a Spear or Bole than to use them. You're right in not wanting to waste the effect of a card, but you also want to have a fresh draw every minute to get you closer to that Balance AoE.
Say you're in transit between packs and while digging you redraw into a Spear. Are you going to use it immediately on someone, having it almost expired by the time you reach the next fight? Are you letting it tick down in your hand until you can use it more effectively, prolonging the time until your next draw? No, you Royal Road that thing.
Say your tank is fighting the last 2 mobs while you redraw into a Bole. Same thing.

And sure, you can use single-target buffs in 8-mans. Having one person buffed is better than having nobody buffed. Your general strategy should be to dig for Balance to spread, THEN start digging for Ewer/Spire to Royal Road. That's because you can still do single-target buffs that way without wasting your AoE on it.
Also, while you already have a Balance/Arrow on Spread, it's generally better to use a Balance/Arrow you draw single-target instead of trying to redraw into AoE.

Another problem I experienced today is 2 AST in 8-man instance. How do I go about buffs overwriting? Prepare AoE Balance (or maybe any of the 3 cards is fine now?) and wait until other AST's buffs fall off? Ofc I'm talking about PUGs with limited or no communication.

Depends. Balance and Arrow are worth more than Spear and Bole. So if you'd end up overwriting the latter with the former, that's fine. If it's the other way around, wait a bit, even if there's only a Balance buff on a single DPS.
And brace yourself for other AST overwriting your buffs with trash all the time because a lot of them don't pay any attention whatsoever.

And while I'm not there yet, are any cards that should be always converted with Minor Arcana?

A card you would normally dump (AoE redrawn into AoE if you've already have it on Royal Road. Bole that's not needed. Unbuffed Spear.) A good DPS buff is worth far more than 300 damage potenty/500 heal potency.

[Elamys]

There's never a reason to go for duration/potency on purpose

Small correction: Duration/Potency is better in 4 mans than AoE.