You could use a fixed value for chapter numwidth
and section indent
in ToC:
\documentclass[a5paper,11pt]{book}
\usepackage{newtxtext,newtxmath}
\usepackage[margin=1.5cm, inner=2.5cm]{geometry}
\usepackage{tocbasic}
\usepackage[bookmarks=true]{hyperref}% load as last package
\newcommand*\chapternumwidthintoc{7em}
\newcommand*\chapterprefixintoc[1]
{\MakeUppercase{\chaptername}~#1\enskip}
\DeclareTOCStyleEntry[
linefill=\bfseries\TOCLineLeaderFill,
beforeskip=0pt,
entrynumberformat=\chapterprefixintoc,
numwidth=\chapternumwidthintoc,
]{tocline}{chapter}
\DeclareTOCStyleEntry[
indent=\chapternumwidthintoc
]{tocline}{section}
\DeclareTOCStyleEntry{tocline}{subsection}
\begin{document}
\cleardoublepage
\phantomsection\addcontentsline{toc}{chapter}{CONTENTS}
\tableofcontents
\chapter{PENDAHULUAN}
contents...
\section{Latar Belakang}
contents...
\subsection{ABCD}
contents
\subsection{EFGH}
...
\section{Rumusan Masalah}
\vspace{0.3cm}
blablabla
\section{Tujuan Penulisan}
\vspace{0.25cm}
blablabla..
\chapter{DASAR TEORI}
contents...
\chapter{PEMBAHASAN}
contents
\chapter{PENUTUP}
contents
\chapter*{BIBLIOGRAPHY}
\addcontentsline{toc}{chapter}{BIBLIOGRAPHY}
contents
\end{document}
Result:

If you still want to use dynnumwidth
for the chapter entry:
Warning: The following suggestion uses an internal command that can be changed or removed in the future. So the code can break in the future.
\documentclass[a5paper,11pt]{book}
\usepackage{newtxtext,newtxmath}
\usepackage[margin=1.5cm, inner=2.5cm]{geometry}
\usepackage{tocbasic}
\usepackage[bookmarks=true]{hyperref}% load as last package
\DeclareTOCStyleEntry[
linefill=\bfseries\TOCLineLeaderFill,
beforeskip=0pt,
entrynumberformat=\chapterprefixintoc,
dynnumwidth
]{tocline}{chapter}
\newcommand*\chapterprefixintoc[1]
{\MakeUppercase{\chaptername}~#1\enskip}
\makeatletter
\AtBeginDocument{%
\DeclareTOCStyleEntry[%
indent=\ifundefinedorrelax{scr@dte@chapter@lastmaxnumwidth}
{1.5em}
{\scr@dte@chapter@lastmaxnumwidth}%
]{tocline}{section}
\DeclareTOCStyleEntry{tocline}{subsection}
}
\makeatother
\begin{document}
\cleardoublepage
\phantomsection\addcontentsline{toc}{chapter}{CONTENTS}
\tableofcontents
\chapter{PENDAHULUAN}
contents...
\section{Latar Belakang}
contents...
\subsection{ABCD}
contents
\subsection{EFGH}
...
\section{Rumusan Masalah}
\vspace{0.3cm}
blablabla
\section{Tujuan Penulisan}
\vspace{0.25cm}
blablabla..
\chapter{DASAR TEORI}
contents...
\chapter{PEMBAHASAN}
contents
\chapter{PENUTUP}
contents
\chapter*{BIBLIOGRAPHY}
\addcontentsline{toc}{chapter}{BIBLIOGRAPHY}
contents
\end{document}
Run three times to get

Example with a greater chapter number:
