@Aichan I agree in part that the artist doesn't know how shields work. Her shield design is garbage. Her shield is shaped like a heater, which should be strapped, but has a centre grip. The centre grip sits behind the shield, instead of more flush with the shields surface, meaning she'll have a lot less control over what the shield does. So lets just assume the centre grip design is fixed (and keep it centre gripped, instead of strapped, since that's how she's using it).
I shouldn't have really mentioned scutums, since they're a horzontal grip, not vertial grip, and both used and designed for different requirements. The Greek hoplon, or aspis, is a variant of a strapped shield, and designed for phalanx warfare (as much to cover the guy to your left as it is to cover you). Formation warfare is also fairly irrelevant to this discussion.
You are 100% correct in that it's easy to knock her shield away, considering the way she's holding it. But it's the
angle she's holding the shield at, not the distance from her body, that's wrong. "but as I said it offers little benefit when you hold it so far in front of you" Viking shields, when used in duels (and the Vikings/Germanic tribes placed a lot of importance in individual prowess), you absolutely held the shield out in front of you, and not "braced against your elbow". They didn't hold it flat on, like she does, but more of a 70 degree angle (point the leading edge at the other guys left shoulder). Holding a Viking shield in close, like you would a strapped shield, defeats the purpose, and utility, of a centre grip.
"What you said was meant for a more agile fighter where trickery and sudden strikes are key, but you can't be agile with that kind of shield." This sounds like RPG nonsense. You don't need much shield between you and them to help obscure what you're doing; this is as true with a buckler as it is with a Viking shield. Also, everything you're saying about heater shields is irrelevant in this instance. She's not using a heater shield, she's using a 'heater shaped' shield with a centre grip. The type of grip is key, since it defines how the shield is used. You cannot use a strapped shield like a centre grip, and vice versa. Her shield is also between the size of a bucker (~30cm diameter) and a Viking shield (~100cm diameter), so while there would be a lot of techniques from each that wouldn't suit her shield, there would also be a lot that are.
I guess we're approaching our critique of her shield use from different directions. I was mostly impressed that they showed her using a centre grip shield in a more or less correct'ish manner (considering it's an isekai manga). Not seeing either the usual video-gamey method of waving your shield around beside or behind you, while hacking away with your sword in front, or statically holding the shield in front of them like a second breastplate, was refreshing.