I need to correct the upper size of the figure (namely to make the rhombus symmetric and the arrow pointing the node s5 to be aligned with the arrows in the lower part of figure) - ignore the non readable text in the last rectangle: its greek language. Thanks
\documentclass[a4paper,twoside,10pt]{memoir}
\usepackage{alphabeta}
\def\eng#1{\textlatin{#1}}
\usepackage{textcase}
\usepackage{silence}
\usepackage{kerkis}
\usepackage{color}
\usepackage[svgnames,x11names,named]{xcolor}
\WarningFilter*{mdframed}{You got a bad break}
\WarningFilter{latex}{Text page}
\usepackage[usetwoside]{mdframed}
\usepackage[LGR,T1]{fontenc}
\usepackage{anyfontsize}
\usepackage{lipsum}
\usepackage {tikz}
\usetikzlibrary {positioning,graphs,calc,decorations.pathmorphing,shapes,arrows.meta,arrows}
\usepackage{graphicx}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsfonts}
\usepackage{amsthm}
\usepackage{mathtools}
\usepackage{systeme}
\usepackage{wrapfig}
\usepackage{afterpage}
\usepackage{tocloft}
\usepackage[figuresright]{rotating}
\usepackage{parskip}
\usepackage{caption}
\usepackage{enumitem}
\usepackage{titletoc}
\usepackage{calc}
\usepackage{pstricks}
\usepackage{bbm}
\usepackage{textcomp}
\usepackage{upgreek}
\usepackage{ifthen}
\usepackage{extramarks}
\usepackage{bm}
\usepackage{multirow}
\usepackage{multicol}
\usepackage{colortbl}
\usepackage[symbol]{footmisc}
\begin{document}
\begin{center}
\tikzset{every loop/.style={min distance=15mm,looseness=10}}
\begin{tikzpicture}[-latex ,auto ,node distance =1.5cm and 3cm, on grid,semithick ,
state/.style ={rectangle ,top color =white , bottom color = cyan!20, draw,cyan , text=blue , minimum width =1 cm}]
\node[state] (s5) {$S5$};
\node[shape=coordinate] (inv) [below =of s5]{};
\node[state] (B) [below left =of inv]{$B$};
\node[state] (s4) [below right =of inv] {$s4$};
\node[state] (t) [below right =of B] {$T$};
\node[state] (d) [below =of t] {$D$};
\node[state] (k) [below =of d] {$K$};
\node[state,yshift=-0.8cm] (pl) [below =of k]
{\begin{tabular}{c}Κλασική Προτασιακή Λογική\\$\diamond\varphi\equiv\neg\Box\neg\varphi$\\Κανόνας Αναγκαιότητας\\Νόμος Απόσπασης\end{tabular}};
\path (pl) edge [bend left = 0] node[xshift=4.2cm, yshift=0.02cm] {$\Box(\varphi\rightarrow\psi)\rightarrow(\Box\varphi\rightarrow\Box\psi)$} (k);
\path (k) edge [bend left = 0] node[xshift=1.8cm, yshift=0.03cm] {$\Box\varphi\rightarrow\diamond\varphi$} (d);
\path (d) edge [bend right = 0] node[xshift=1.6cm, yshift=0.03cm] {$\Box\varphi\rightarrow\varphi$} (t);
\path (t.north) edge [bend right = 0] node[yshift=0.03cm] {$\varphi\rightarrow\Box\diamond\varphi$} (B);
\path (t.north) edge [bend right = 0] node[xshift=2.1cm, yshift=-0.505cm] {$\Box\varphi\rightarrow\Box\Box\varphi$} (s4);
\path (B) [-] edge [bend right = 0] node[below =0.15 cm] {} (inv);
\path (s4) [-] edge [bend right = 0] node[below =0.15 cm] {} (inv);
\path (inv) edge [bend right = 0] node[xshift=2.05cm, yshift=0.03cm] {$\Box\varphi\rightarrow\diamond\Box\varphi$} (s5);
\end{tikzpicture}
\end{center}
\end{document}
