Is it possible to customize the default behavior of sagetex?

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You can follow the strategy given in my answer to this question you made later. For the sake of completeness, I adapt it here.

Write a tex file with all the code you need to config SageTeX:

\begin{sagesilent}
   latex.matrix_delimiters(left='[', right=']')
   latex.vector_delimiters(left='<', right='>')
\end{sagesilent}
\endinput

Call it, for example, STconfig.tex. Then write a small package mysagetex.sty with the following content:

\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{mysagetex}[2019/02/20 v.0.1]
\RequirePackage{sagetex}
\AtBeginDocument{\input{STconfig}}
\endinput

Put both files together where TeX can find them. In the source tex file, it suffices to load mysagetex instead of sagetex, e.g., add \usepackage{mysagetex} to the preamble.