The problem is that the mud comprises Tiamats saintgraph ( and she can use more of it to make herself stronger), if you know what a saint graph is (you probably do), you can start to see the issue.
Eh, it's not really a problem. It's a part of herself, but it's not the "main" part of her, nor is it her strongest ability. It's just one of her Authorities. I also don't remember her using it to power herself up. She uses it to overwrite Ereshkigal's Underworld, but that's not exactly the same thing.
Also, look at my Goku example. He uses his own Ki to power himself and his attacks, which have 4D attack potency despite being 3D. Actually, Vegeta might be a better example since, if I recall correctly, he achieves Super Saiyan God Super Saiyan, skipping Super Saiyan God, just by training himself really hard (no ceremony needed).
The narration later says that kick was done by brute force
The actual name of the attack is "Flare Skirt Bunker Buster" with the alternate reading being "Manifestation of a Woman's Power, Akin to a Solar Flare," which doesn't necessary mean the attack is as strong as a solar flare.
Plus, we don't take attack names into account unless there's some other stuff that backs it up, unless you want to argue that manga Android Saga Vegeta is 3-A to Low 2-C because he uses an attack called "Big Bang Attack."
Also, if we did did take into account the alternate readings, we'd have to give Excalibur some sort of fate manipulation since it's the "Sword of Promised Victory" or go for 2-B Archer Oda Nobunaga since her Noble Phantasm, Three-Line Formation, has the alternate reading "Three Thousand Worlds." (Actually, we should do that, that sounds great).
Goetias big beam is even said to be able to be tanked by it, and that thing scales above 99% of the verse
That's mostly because Tiamat is similar in strength to Goetia, and because Tiamat's immortality is bullshit incarnate. There are very few characters in the franchise that could actually kill her.
My king.
Technically this could also be MMC Gilgamesh btw.
Eh, probably not since Gilgamesh is considered Classless in Fate/Extra CCC.
Literally the same thing I said
I know, I just didn't see it when I typed out my response.