How to make good-looking linkages between single and double bonds in chemfig?

See chemfig manual, section Shifted double bonds:

All double bonds are made up of two line segments, and these segments are drawn on either side of the imaginary line along which a single bond would be drawn. It is possible to shift a double bond so that one of the line segments lies on the imaginary line. The other segment is then shifted above or below the bond. Actually, it is more correct to say “left” or “right” of the imaginary line, as the bond is traversed in the direction of drawing.

To shift the bond to the left, write “=^” and to shift it to the right, write “=_”:

Applied to your MWE:

documentclass{article}
usepackage{chemfig}

begin{document}
    
setchemfig{bond join=true, atom sep=2.5em, bond style={line width=1.pt}}
chemfig{A-=_[:330]B}
chemfig{A-=^[:330]B}
    
end{document}

P. S. If you want to make the structure look IUPAC-compliant, you need to invest some extra work as your style and chemfig defaults are a bit off general recommendations [1, 2]. Same goes for the famous "ACS Document 1996" style (ACS author guidelines (PDF)).

References

1. Brecher, J. Graphical Representation of Stereochemical Configuration (IUPAC Recommendations 2006). Pure and Applied Chemistry 2009, 78 (10), 1897–1970. DOI: 10.1351/pac200678101897. (Free Access)
2. Brecher, J. Graphical Representation Standards for Chemical Structure Diagrams (IUPAC Recommendations 2008). Pure and Applied Chemistry 2009, 80 (2), 277–410. DOI: 10.1351/pac200880020277. (Free Access)