Plate + 1 isn't too much. Think long term. By the time you reach Minthara, you are almost certainly level capped if you've played out most of or all of the side quests, etc. So, you should actually be roughly Level 5 by the time you face her. I understand that this is open world and you COULD get to her before that, but if you are foolish enough to face her at level 2 or 3, that's on you. In most video games, there is an end boss you have to grind up levels to beat. Minthara, Ragzlin and Gut should be those characters in EA. Yes, you can bypass them altogether if you want. Absolutely. But that just means even more so that she and others should be made to fit the story and not necessarily what level and/or equipment the characters have.
And you are going into the UNDERDARK!!! You absolutely should be able to have at least one character with Plate + 1 if you did something as awesome as kill off the bad guy goblin leaders. It would be a nice end quest reward.
And let's face it, not every character can use Plate Mail. You have to have Heavy Armor proficiency and Strength 15, so even if Minthara's armor reduced the Strength prerequisite to 14, which isn't necessary since she has Strength 15, only Fighters and Clerics could even use the armor. So if you're playing a Rogue, for example, you might, maybe, give the suit of armor to Lae'zel. That's it. Lae'zel might then prove to be a valuable asset to your journey in the Underdark where you face Minotaurs, the Bullette, etc.
