There are several reasons why you might want to cast True Strike. The first is that depending on your class, you might be able to get a bonus action attack after casting it, giving you one attack with advantage and one without, a definite improvement on two attacks without.
The second is that you might use it to overcome disadvantage:
- When you need a die result of 9 or higher to hit, a single normal attack becomes more likely to hit than 2 attacks with disadvantage, so it's worth the trade-off of using True Strike.
- When you need 8 or below, though, two attacks with disadvantage are more likely to hit, so just attack away without True Strike.
The third reason is that you might want advantage for something. For example, a Rogue might use True Strike to grant advantage to allow use of Sneak Attack.
Another reason is that you might know in advance that there was going to be combat, and cast True Strike for advantage on the first round. This would probably only happen when ambushing, but is still worth mentioning.
The final reason is that you might have an attack that you particularly wanted to hit. A Wizard using Plane Shift to send an enemy to the Abyss, for example, would want to be certain that the melee spell attack required would hit, otherwise they've wasted a high-level spell slot for nothing. Casting True Strike first would make it considerably less likely to miss.
Your ruling is correct.
The effect of shield ends at the start of your next turn, which is the next turn you take. Your ruling #1 is correct.
There are many spells which have a listed duration of 1 round, 1 minute, 1 hour, or so on with a trigger listed in the text that causes the spell to end before that duration is reached. Although we might prefer the spells listed their duration as "up to 1 round," "up to 1 minute," or so on, the implication is indeed that that duration is a maximum limit. So the listing of a 1 round duration does not contradict the listing of an ending condition triggered by the start of your next turn.
Justifications for this ruling.
Each time a creature's turn ends, the next creature's turn begins. So your next turn is the occurrence of your creature acting following the last turn you took, whether or not a full round of turns has elapsed. This is the logical consequence of Jeremy Crawford's clarification on the flow of turns (see this tweet):
When your turn ends, the next person's turn starts.
Without going into a needlessly formal proof using the well-ordering principle, Crawford's statement establishes that your next turn is defined by the ordering of the turns as they elapse, not by the quantity of turns that have elapsed.
Also, a stated design intent in 5e is that words should be read according to their idiomatic meanings. In English, "your next turn" means "your next turn," not "your next turn or after 1 round, whichever is sooner." The simplest interpretation is both intended and correct.
Why it is a good Level 1 spell.
Your friend's reasoning that shield is weak is fairly unsound, and ruling #2 is neither necessary nor supported by the rules.
It's not that you're losing future time on the duration of the spell if you cast it too late in the round before the start of your next turn. You are de facto protected from all attacks for a whole round between any one of your turns and your following turn by (A) the reaction casting of shield mid-round, by (B) the ongoing spell effect after that until your following turn, and by (C) the threat of what we might call "potential shield" before it was actually cast (since before you cast shield there were probably no attacks that targeted you anyway and for the entire round enemies may have chosen not to attack you due to knowledge that you possess the potential to cast the shield spell).
In my experience in games in which I have been a player or a DM, squishy casters like Wizards have used it at least once but usually twice per session. Now note that a +5 bonus to AC is more than what any mundane armor or rare magical armor can confer and it can often be relied upon for most of the attacks that target you for a whole encounter due to the spell slot and action economy. You even get to wait and see if an attack would hit you before you choose to use it.
Therefore, I assert that it is a plenty powerful and comparable Level 1 spell with good utility. It does not need to be fixed. Stick with your correct ruling.
Best Answer
The way I've interpreted this is that the spell itself, the casting of it, takes the duration of one round, your current action. The effect itself does not take place until your next turn as stated in the spell text proper. Once your next turn begins the spell is active, so to speak, and for the duration of that round you gain the advantage listed.
Furthermore if concentration is lost between casting and actual use, you'd resolve it as you would normally for checking on a break in concentration.
So cast spell, wait for next turn, spell effect is active for that turn, profit.
Furthermore I'd treat an attack on that turn that a PC might be able to make as a bonus action as the "next turn" attack with advantage, aka the first attack roll against the target mentioned in the spell proper. But the text is pretty clear that it starts working on your next turn, so that would likely be up to the DM.