Extremely dumb shit that Narita shamelessly enables me to saySo, Yuukaku started his magecraft project because he found some Age of the Gods bacteria in South/Meso-American ruins.
Quetzalcoatl lore explains that Aztec gods are bacteria that could only manifest by infecting and possessing humans and that the Divine Spirit Servant Quetzalcoatl we know is just one of the random human bodies infected with the real Quetz. Despite this, that woman is still fully counted as an Aztec god by the laws of the setting. Therefore, at the current moment, we have no reason to believe Tsubaki doesn't fit the series' official definition of Aztec god. Logic dictates Tsubaki's body should just as summonable as Divine Spirit vessel as the body Quetz uses in FGO.
Daybit's Servant has been already hinted to be Tezcatlipoca, so until proven otherwise, I am going to believe that Tsubaki Kuruoka is the Grand Berserker.
On a slightly more serious note (make no mistake, this is still a shitpost), since Tsubaki’s unintentionally summoned Servant had vague death god attributes, she also could very likely to be Mictecacihuatl.