The more I think about it, the more I'm persuaded the only meaning of the serum bowl is to manifest at the reader's eyes how boundless and unfathomable the love Levi felt for Erwin was.
Paradoxically speaking, If Levi had titanized Erwin, ppl would have discharged it justifying it as an "act out of duty".
However, precisely for he accepted to embrace the choice that was against all his reasons and his believings, that ppl should question the motivation behind it