You get a free reset when you Job Advance at level 80. It doesn't matter which way you choose.
Personally, I prefer the offensive build instead of learning the Wizard's heal skills. Those heal for a pretty low amount, which pots can easily help recover. In a mobby situation, pots > Wizard heal since you stand still when casting it.
For FS, most players tend to get Gumiho for the Critical Damage increase. If your BF is willing to let you use the Tengu, you could use that instead. Tengu is nice in that it provides you with +11% Max Physical/Magical Damage and +6% Minimum Physical/Magical Damage at level 20.
Priring and Black Cat are nice but not in a PVE sense. Both are geared more towards PVP since Priring increases Accuracy and Black Cat increases Evasion.
Yes Cure Poison/Cure Disease is useless.
No one knows what Cure Disease does. Cure Poison is pointless because most monsters that use Poison can Poison pretty fast once the previous one has ended.
As a Wizard, you only need Magic Upgrade.
When you advance into a Bard, you'll still get Magic Upgrade but what tree you go down depends on what type of Bard you want.
Elemental Flow's tree is nice in that you regain HP/SP at random when you successfully landed a magical hit. Crit tree is to increase your Crit Rate/Crit Damage. Purification Tree is independent and that's mostly for healing support.
You can look through the Bard Skills
on our Wiki to see which path you might like.