Rotated bold font in NiceTabular table heading

Question

I am making table using both nicematrix and makecell packages, because these packages applied together allow to obtain multirow heading cells containing rotated text (as pointed here; if I am not wrong, makecell package alone enables making cells with rotated text, enables merging vertically adjacent cells, but not both for a single cell). I need to highlight table-heading content by setting not only gray background, but also bold font. I have two problems/questions:

1. The content of \Block is shifted upward from the cell center - letter Ś touches top bounding line of the cell (see picture below); and letter m in the cell with rotated text almost touches the corresponding bounding line of the cell. How to center \Block content automatically?
2. Does the \Block command enable for application bold footnotesize font for a group of cells, like \renewcommand{\theadfont}{\footnotesize\bfseries} in makecell package?

Code:

\documentclass[table]{standalone}
\usepackage{siunitx}
\usepackage{makecell}
\usepackage{nicematrix}
\renewcommand{\arraystretch}{1.3}
\begin{document}
\begin{NiceTabular}[hvlines,code-before=\rowcolor{gray!30}{1-3}]{%
    S[table-format=2.1]
    S[table-format=1.2]
    S[table-format=2.1]
}
\multicolumn{2}{c}{\makecell{Średnia wartość\czegoś tam}} & 
{\Block{2-1}{\rotate \makecell{Obrócony opis\kolumny B}}} \
{\Block{2-1}{\rotate \makecell{straszne dane}}}
& \rotate \makecell{inne dane\z długim opisem} \
& {A} & {B} \
63.5 & 4.87 & 63.5\
88.4 & 5.13 & 88.4\
\end{NiceTabular}
\end{document} 

Answer 1

Your problems are not linked to the command \Block (provided by nicematrix) since the command \Block is not used in both cells with problems (however, using \Block in those cells won't solve the problems since the command \Block does not create space).

In fact, your problems would arise in a standard {tabular} of LaTeX (provided you use explicitely \rotatebox instead of the syntactic variant \rotate provided by nicematrix).

---

For the first cell (whose content is Średnia wartość czegoś tam), I suggest to use the command \gape provided by makecell since you have used the command \makecell.

That gives : \gape[t]{\makecell{Średnia wartość\\czegoś tam}}.

---

\documentclass{article}
\usepackage{siunitx}
\usepackage{makecell}
\usepackage{nicematrix}
\renewcommand{\arraystretch}{1.3}
\begin{document}
\begin{NiceTabular}[hvlines,code-before=\rowcolor{gray!30}{1-3}]{%
    S[table-format=2.1]
    S[table-format=1.2]
    S[table-format=2.1]
}
\multicolumn{2}{c}{\gape[t]{\makecell{Średnia wartość\czegoś tam}}} & 
{\Block{2-1}<\rotate>{Obrócony opis\kolumny B}} \
{\Block{2-1}<\rotate>{straszne dane}}
&{\Block{}<\rotate>{inne dane\z długim opisem;}}\
& {A} & {B} \
63.5 & 4.87 & 63.5\
88.4 & 5.13 & 88.4\
\end{NiceTabular}
\end{document} 

Answer 2

For the blocks, the text is already at the center. If you want to shift further, use \hspace to achieve the desired output. For the spacing, you can try adjusting the cell width else reduce the size of the text, so that the text will not touch the cell border. This can be achieved by using \small or \scriptsize. Just use it before the text.

\documentclass[table]{standalone}
\usepackage{siunitx}
\usepackage{makecell}
\usepackage{nicematrix}
\begin{document}
\centering
\begin{NiceTabular}[hvlines,code-before=\rowcolor{gray!30}{1-3}]{%
S[table-format=2.1]
S[table-format=1.2]
S[table-format=2.1]
}
\multicolumn{2}{c}{\makecell{\scriptsize{Średnia wartość}\\scriptsize{czegoś tam}}} &
{\Block{2-1}{\hspace{-.4cm}\rotate \makecell{Obrócony opis\kolumny B}}} \
{\Block{2-1}{\centering\rotate \makecell{straszne dane}}}
& \rotate \makecell{inne dane\z długim opisem} \
& {A} & {B} \
63.5 & 4.87 & 63.5\
88.4 & 5.13 & 88.4\
\end{NiceTabular}
\end{document} 

Edited answer after comment:

\documentclass[table]{standalone}
\usepackage{siunitx}
\usepackage{makecell}
\usepackage{nicematrix}
\begin{document}
\centering
\begin{NiceTabular}[hvlines,code-before=\rowcolor{gray!30}{1-3}]{%
S[table-format=2.1]
S[table-format=1.2]
S[table-format=2.1]
}
\multicolumn{2}{c}{\makecell{\scriptsize{\textbf{Średnia wartość}}\\scriptsize{\textbf{czegoś tam}}}} &
{\Block{2-1}{\hspace{-.4cm}\rotate \makecell{\small \textbf{Obrócony opis}\\small\textbf{kolumny B}}}} \
{\Block{2-1}{\centering\rotate \makecell{\small \textbf{straszne dane}}}}
& \rotate \makecell{\scriptsize \textbf{inne dane}\\scriptsize \textbf{z długim opisem}} \
& {A} & {B} \
63.5 & 4.87 & 63.5\
88.4 & 5.13 & 88.4\
\end{NiceTabular}
\end{document}