Forest: classic decision tree

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edit: an attempt to make tree code slightly shorter

• from preamble of your mwe is relevant only package forest
• styles of nodes and edges labels are defined as for tree options
• style for labels is moved in forest environment
• to tree code are added comments for short its explanation

\documentclass[margin=3mm]{standalone}
\usepackage{forest}

\begin{document}
\begin{forest}
/tikz/every label/.append style={xshift=-1pt,inner sep=1pt,font=\footnotesize},
  for tree={
    grow'=east,
  % node style
    line width=0.4pt,
    minimum size=2pt,
    inner sep=2pt,
    if n children=0{}{draw}, % <--- added
  % anchors <------- solve basic problem 
    parent anchor=children,  
    child anchor=parent,     
  % distances of nodes
      l sep=17mm,           % <-- *l*: level distance
      s sep=2mm,            % <-- *s*: sibling distance between leaves
  % edges
    edge={draw},
    edge path'={
       (!u.parent anchor) -- ([xshift=-13mm].child anchor) -- (.child anchor)
                },
  % labels on edges
    delay={
      edge label/.wrap value={node[midway, inner sep=1pt,
                                   font=\scriptsize, above]{#1}},
           },
  }, % end for tree
  [ , label=$n_0$
    [, label=$n_3$, edge label={$s_5,s_6$}
      [$o_6$, edge label={$s_6(n_3)$}]
      [$o_5$, edge label={$s_5(n_3)$}]
    ]
    [ , label=$n_2$, edge label={$s_3,s_4$}
      [$o_4$, edge label={$s_4(n_2)$}]
      [$o_3$, edge label={$s_3(n_2)$}]
    ]
    [ , label=$n_1$, edge label={$s_1,s_2$}
      [$o_2$, edge label={$s_2(n_1)$}]
      [$o_1$, edge label={$s_1(n_1)$}]
    ]
  ]
\end{forest}
\end{document}