So now that Trailblazer is at the training grounds a question arises. Would I be very wrong to assume that performing rolls related to certain attributes, like [STR] for hitting practise dummies or perhaps reading many books with [INT], would increase those with enough turns? Or are attribute changes reserved for item properties and magic only?

Like a cutie mark, a character's base attributes do not change. However, it is possible to adjust their values by equipping items with attribute modifiers. As indicated by the attribute in the item's description, brass horseshoes are a Strength-based weapon, meaning that they were designed to increase the impact of stronger attacks. Note that a higher attribute score increases the ease of use of a weapon, not the effectiveness. Unlike most weapons, anyhow, horseshoe weapons have a special mechanic that allows the wearer to attack using both strength with more powerful blows, or dexterity with quicker and more precise jabs. It's important to adjust the fighting style accordingly.