Flow chart diverging and converging arrows

Edit: In diagram are added new nodes as you require in edited question.

In MWE (Minimal Working Example) below are in comparison to your code fragment done the changes:

• nodes are organised in three chains (using TikZ library chains) where is for positioning of nodes used positioning library and its syntax (by this distances between nodes are equal in determined by node distance = <below> and <right>)
• nodes in chains are linked by arrows drawn by instruction join defined in the chains package
• connections between nodes are discontinued in four places by use of code suspend join
• connections between nodes, which are not considered in join, are drawn separately

documentclass[tikz, margin=3mm]{standalone}
usetikzlibrary{arrows.meta,
                chains,
                positioning,
                shapes.geometric}

begin{document}
    begin{tikzpicture}
makeatletter
tikzset{
  node distance = 4mm and 2mm,
    start chain = A going below,
     arr/.style = {-Stealth},
    base/.style = {draw, fill=blue!30, font=small,
                   text width=42mm, minimum height=8mm,
                   align=center},
   block/.style = {base, rounded corners},
decision/.style = {base, diamond,  aspect=1.6, 
                   text width=21mm, inner xsep=0pt},
suspend join/.code={deftikz@after@path{}}
        }
makeatother
% Place nodes
% main branch
    begin{scope}[nodes={block, on chain=A, join=by arr}]
node   {Get Dante AV board from batch};            % name: A-1
node   {Install fan, heat sink, 
         thermal dough and 4 jumpers on the board};  
node   {Complete Voltage Testing};
node   {Record serial number Voltage Testing 
         is a pass or fail to spreadsheet};         % name: A-4
    end{scope}
coordinate[below=of A-4] (aux1);
    begin{scope}[nodes={block, on chain=A, join=by arr}]
% first left branch
% here had to be discontinued "join" instruction from main branch
node [suspend join,                         % name: A-5
       below  left=of aux1 -| A-4.west] {Flash as Tx};
node                                   {Test Tx with textit{Golden Rx}};
% first right branch
% here had to be discontinued "join" instruction from main branch again
node [suspend join,                         % name; A-7
       below right=of aux1 -| A-4.east] {Flash as Rx};
node                                   {Test Rx with textit{Golden Tx}};
    end{scope}
% decision
coordinate[below=of A-6.south -| aux1] (aux2);   
node [decision, below=of aux2] (decision)      {Pass/Fail};
% end branches
    begin{scope}[nodes={block, on chain=A, join=by arr}]
% left end branch
% here had to be discontinued "join" instruction from main branch again
node [suspend join,                          
       below=of decision -| A-6]    {Flash P};  % name A-9
node                               {Test T};
node                               {Record};
% right end branch
% here had to be discontinued "join" instruction from left end branch
node [suspend join,                          
       below=of decision -| A-8]    {Flash F};  % name A-12
node                               {Test T};
    end{scope}
% Draw edges which are not considered in join
% left
draw[arr]  (A-4) -- (aux1) -| (A-5);
draw[arr]  (A-6) |- (aux2) -- (decision);
draw[arr]  (decision) -| (A-9)     node[pos=0.25,above] {Pass};
% right
draw[arr]  (aux1) -| (A-7);
draw       (A-8) |- (aux2);
draw[arr]  (decision) -| (A-12)    node[pos=0.25,above] {Fail};
    end{tikzpicture}
end{document}

produce:

Is this what you after?