Is there a way to employ auto-completion for Biblatex commands (not entries) in Texmaker without writing all commands in User Customization Auto-Completion manually?
If you want TeXmaker to autocomplete commands that it does not already know, you need to add these commands manually via User > Customize Completion", see How can I make texmaker autocomplete commands that it doesn't already autocomplete?. To add
\autocite for example you could add
in that dialogue window.
TeXmaker also automatically adds all
.bib entries it can find to the autocomplete list (note that not all
.bib files are found automatically, if they reside in a different folder from your
.tex file, you may have to tell TeXmaker about that even though BibTeX/Biber can find the
.bib file just fine).
TeXstudio (a fork of TeXmaker if I understand correctly) seems to have slightly more sophisticated autocomplete feature. It can automatically detect which set of auto-complete commands to provide. The
biblatex autocomplete list seems very comprehensive and already has all the common commands available.