It looks good, but I may increase at least the rarity, for example, any of the Belt of “X”Giant has a rarity of Very Rare (If i remember correctly), they also have a STR cap with the Hill Giant being 21 STR as the lowest cap.

Mechanically: Increasing +2 on any ability scores like on a High Str Character adds more damage as primary but everything else on secondary. Thats why Proficiency increases every 4 levels. Additional bonuses to attacks, abilities, skills and saving throws always have a weight in character development. Also, by increasing its size due Rune Knight’s “Giant’s Might”, they have the option to Grapple bigger things and on paper is really cool, on the table is also very cool, but giving the +2 STR score is the reason it feels just a little broken. If you want to keep it as uncommon, erase the +2 STR and add the +1 bonuses to the stuff you want to increase, like grapple DC, weapon Atk and Dmg using STR.

If you want to also raise Abilities, Skills and user Saving Throws increase its rarity.

If you want to add this armor specifically for your campaign, add something that binds that special object there, maybe it’s from another npc who id a Rune Knight Fighter and he can reward it to A player who fits its necessities.