As you have more or less any permutation with brackets and sequence in your tags, you have to deal with every case individually.
What I would suggest:
First copy the contents of TITLE to FEAT_ARTIST
(FEAT_ARTIST is now a user-defined field). You achieve this with an action of the type Format value for FEAT_ARTIST and the format string %title%.
Now apply several actions of the type "Guess value" for FEAT_ARTIST
Each of these actions cuts away the stuff that you don't want, e.g.
Title (feat. Artist) (Mixartist Mixtype)
can be extracted with
%dummy% (feat. %feat_artist)%dummy%
I am sure, you can devise similar actions for other sequences or brackets.
Once you have extracted the featured artist to FEAT_ARTIST, you can now append ARTIST with FEAT_ARTIST with an action of the type "Format value" for ARTIST and the format string
%artist% (feat. %feat_artist%)
After that you delete the field FEAT_ARTIST.
And then you delete any bracket followed by "feat." with an action of the type "Replace with regular expression" for TITLE, e.g.
Search: (.) (feat.)\s(.*)
Replace: $1 $2