Can't reference theheading in class titleformat commands

I’m writing a LaTeX template.cls and am trying to redefine how the chapter, section and other headings are shown in the file. To do this, I’m using titleformat from titlesec to accomplish this. Here’s the code I’m currently using in my template file:

RequirePackage{titlesec}
    titleformat{chapter}[block]
                {normalfontbfseriesHuge}
                {{chaptertitlename}~arabic{chapter}:}
                {0pt}{}[]
    titleformat{section}[block]
                {normalfontbfserieshuge}
                {Sarabic{chapter}.arabic{section}:~}
                {0pt}{}[]
    titleformat{subsection}[block]
                {normalfontbfseriesLarge}
                {arabic{chapter}.arabic{section}.arabic{subsection}:~}
                {0pt}{}[]

This block works nicely as expeced. For example, the following block will produce the image’s output:

chapter{The tale of the chapter}
section{Starring: The section}
subsection{Directed by: The subsection}

My problem is that, while this is a good implementation, it interferes with the appendix package (and surely others) in that the chapter is defined as an arabic number and can’t be changed into an alphabetic index. So, even though the appendices’ sections are shown in the table of contents as [A-Z].[1.9], they’re shown as [1-9].[1-9] in the actual document.

To fix this, I’m trying to define the title format like this:

titleformat{chapter}[block]
            {normalfontbfseriesHuge}
            {{chaptertitlename}~thechapter:}
            {0pt}{}[]

However, I’m getting compilation this error:

! Undefined control sequence.
<argument> {chaptertitlename } thechapter: 
                                             
l.29 chapter{The tale of the chapter}

I’ve also found that the problem is not that thechapter isn’t usable in the whole *.cls file, but that it’s not usable in that particular block (maybe because it’s referencing its own heading?). For example. the following doesn’t produce any compilation errors and produces the same output as the image above:

titleformat{section}[block]
            {normalfontbfserieshuge}
            {Sthechapter.arabic{section}:~}
            {0pt}{}[]

However, using thesection in titleformat{section} produces a compilation error:

titleformat{section}[block]
            {normalfontbfserieshuge}
            {Sthesection:~}
            {0pt}{}[]

! Undefined control sequence.
<argument> S thesection: 
                           
l.30 section{Starring: The section}

I truly am at a loss here. What am I doing wrong? Should I be using a reference other than the*? Have I found LaTeX’s ceiling? What can I do to make this work? Thank you very much!