Simple answer: if you're going to wear less armor, use a shield.
Longer answer: In melee fighting, from my (limited) experience there are three places you want to cover:
The first two are places you don't want a sharp object to go, and the third is usually going to end up exposed and close to the enemy. Historically, when the money became available soldiers would get helmets, then some form of chest armor (maille/gambeson/cuirass).
But it sounds like that's not something you want to do. You want the chest exposed, and probably the face/hair too. So I think your best option is to use a shield.
Historically, there are accounts of people fighting naked. It's not usually a good idea, but it becomes a less suicidal option if you have a shield. Shields vary in size and shape; they can be as large as a man, or as small as a dinner plate. But even the smallest shields carry great defensive capabilities, especially to those who know how to use them. If you have good armor, you don't need a shield; but on the other hand, if you have a shield, you have less need for armor.
A buckler (the plate-sized kind of shield), when held at arm's length, can cover someone's chest and face. Used skillfully, it can cover the entire body (just not all at once). If your character wants to have freedom of movement, then I think removing all armor and going all-out for a light, maneuverable buckler might be her best option.
After the buckler, if you still want armor, I think the thigh armor makes sense. Covering the lower legs would be a good idea too, but the further you move from your body's core, the heavier the armor is going to feel, so I'll leave that up to you. But basically if you don't have to move the buckler to guard your legs, you can keep it up to guard your face. The same goes for armoring the non-shield arm, but it's less necessary because you can use the buckler there.
The only issue I see in your case is that it's hard to use a polearm one-handed. But like you said, this character can increase her strength, and you can probably design a buckler such that you can still use that hand to grasp the polearm.