If Barkskin raised AC by 2 without concentration until long rest it would be OP to the point of effectively being mandatory when you have a druid in the party. Especially as you could then cast it on every single party member each day.
Raising AC by 2 *with* concentration until long rest is maybe balanced. (essentially an unlimited duration Shield of Faith, at the cost of a 2nd level spell slot.)
Raising AC by 2 *without* concentration for 1 minute is also maybe balanced, but probably less so.
Changing Barkskin to be a 1st level spell makes it too similar to SoF to warrant being its own spell.
I'd set it to work on a timer (50 turns). One ally only. All of the druid's good level 2 spells work the same way. I'd hate seeing the druid turned into a full blown support. However, this way they can spend one slot on support and one slot on damage.
Depending on how weak the wild shapes remain for tanking, it also improves the moon druid.
The old problem remains that killing faster is better than +2AC almost always. I don't think this is OP simply because of how much damage you're losing. A druid is wiped out at level 4 in two (moonbeam) to one fights (flameblade). Introducing barkskin means one cast at even level 5 per day. Since this class is low-key bad at the moment, it needs a good spell or two.
Barkskin fits in neatly to make them a more viable tank in parties with no alternative. It gives them one thing an ancient paladin cannot do in practice. The ancient paladin will always have more damage and healing... that only leaves defense.
Even so, ancients can buff their AC permanently by 1. Though, it means losing out on damage. But, they can do that... and still have permanently more damage than the druid. RIP.
(I don't think this needs saying, but druid concentration also makes no sense. You'll sooner die than lose moonbeam, yet flameblade disappears when an enemy sneezes nearby. What does that mean for barkskin?)