Let me illustrate my comment/proposal about [continuous refactoring][1].
## 1. Step - rebuilding your example - V1
Remember? Start somewhere. Here: rebuilding what you posted for compiles. So it's a useful illustration of the *process*.
### Preparations
I took the liberty to:
- simplify your **main.tex** a little bit
- put all your files and folders into folder V1 (see screenshot)
- add more versions later, which match the numbers/versions in my Refactoring comment (in the **main.tex** versions)
[![Refactoring folders][2]][2]
You can use a similar versioning approach (copy folder Vn into V(n+1)), use Git etc. Versioning and backup are certainly on your todo- or done-lists.
So, this is what my **main.tex** looks like in `V1`, for a simplified start:
~~~
% ~~~ REFACTORING ~~~~~~~~~~~~~~~~~~~
%
% 1. getting copies to compile; introducing some formatting
% activated 1.x and 2.x, dropped bibliography and appendices
\documentclass{report}
\usepackage{geometry}
\usepackage{lipsum}
%\usepackage[backend=biber,style=ieee]{biblatex}
%\addbibresource{bib.bib}
\usepackage[colorlinks]{hyperref}
% ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\begin{document}
\input{titlepage/titlepage}
\tableofcontents
\input{1.a/a}
\input{2.b/b}
%
%\printbibliography[heading=bibintoc]
%\listoffigures
%\addcontentsline{toc}{chapter}{List of Figures}
%\listoftables
%\addcontentsline{toc}{chapter}{List of Tables}
%
%\appendix
%
%\input{appendix/appendixA}
%\input{appendix/appendixB}
\end{document}
~~~
All needed **.tex** files are as you posted, within this folder structure:
~~~
+- main.tex
+- titlepage/
| +- titlepage.tex
|- 1.a/
| +- a.tex
| +- a1.tex
| +- a2.tex
|- 2.b/
| +- b.tex
| +- b1.tex
| +- b2.tex
~~~
### MY first refactoring-action
Preferences, tastes, perception etc. do vary from author to author. For me I'd already perceive your main-code as a bit crowded and hard to tell, what goes where. So I followed *my* addiction, to put some code-formatting, see above.
Now, after enough compiles, looking ahead, my first attention point was the titlepage-file ... it should be different in my view (still just for demo-purposes).
To *emphasize*, I line out the *way of thinking*, the *self-improving process*, and less the code itself.
## 2. Step - (first) adjustement of titlepage.tex - V2
Well, in **main.tex** nothing changed, besides my comment/protocol:
~~~
% ~~~ REFACTORING ~~~~~~~~~~~~~~~~~~~
%
% 1. getting copies to compile; introducing some formatting
% activated 1.x and 2.x, dropped bibliography and appendices
% 2. refactoring titlepage
~~~
While in **titlepage.tex** I took the liberty to
- introduce the titlepage-environement
- add the folder structure (for fanciness)
:
~~~
\begin{titlepage}
Fancy title:
\vspace{1cm}
\lipsum[1-2]
\vspace{2cm}
Folders and files used:\bigskip
\begin{verbatim}
+- main.tex
+- titlepage/
| +- titlepage.tex
|- 1.a/
| +- a.tex
| +- a1.tex
| +- a2.tex
|- 2.b/
| +- b.tex
| +- b1.tex
| +- b2.tex
\end{verbatim}
\end{titlepage}
~~~
There are millions other ways to design this one ...
[![titlepage][3]][3]
## 3. Step - enabling Chapter 1 to be compiled in two ways:
1. within **main.tex**
2. separately as **a.tex**
### Required changes in main.tex
~~~
% ~~~ REFACTORING ~~~~~~~~~~~~~~~~~~~
% V# changes
% --- -----------------------------------------------------------
%
% 1. getting copies to compile; introducing some formatting
% activated 1.x and 2.x, dropped bibliography and appendices
% 2. refactoring titlepage
% 3. adding subfiles to chapter-1 file a.tex, so can be compiled alone
\documentclass{report}
\usepackage{subfiles} % <<< NEW
\usepackage{geometry}
\usepackage{lipsum}
%\usepackage[backend=biber,style=ieee]{biblatex}
%\addbibresource{bib.bib}
\usepackage[colorlinks]{hyperref}
% ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\begin{document}
\input{titlepage/titlepage}
\tableofcontents
% \input{1.a/a}
\subfile{1.a/a} % <<< CHANGED
\input{2.b/b}
%
%\printbibliography[heading=bibintoc]
%\listoffigures
%\addcontentsline{toc}{chapter}{List of Figures}
%\listoftables
%\addcontentsline{toc}{chapter}{List of Tables}
%
%\appendix
%
%\input{appendix/appendixA}
%\input{appendix/appendixB}
\end{document}
~~~
This lets you compile the whole thesis:
[![thesis][4]][4]
### Chapter 1 as a pdf
**a.tex**:
~~~
\documentclass[../main]{subfiles} % <<< new
\begin{document} % <<< new
\chapter{First}
\input{1.a/a1}
\input{1.a/a2}
\end{document} % <<< new
~~~
Compiles into, which might be useful for separate inspection, review, crtics:
[![ch1][5]][5]
## 4. Process review and outlook
The 3 steps I showed here are already `refactoring`, like doing pottery on content:
- start simple, with a goal in mind
- gradually transform and check the outcome(s)
Wrt. your thesis I can think of a number of scenarios, like:
- splitting some part of say **a.1.tex** into more subparts (growing, segmenting)
- consolidating: perhaps later you feel, all **b*.tex** files should be in one **b.tex** (growning, merging)
- you recognize, a separate `tikz/` folder with `standalone` drawings is useful
- later you do some finer segmentation on it, like `tikz/ch1/`, `tikz/ch2/` etc.
- and so on
`Refactoring` in itself has no limits. Above I showed impact on the segmentation into `files and folders` you presented. You can also do the same with `content`, i.e. your messages and findings. And as indicated you can refactor the `folders`. E.g. after some cycles you may have ended up like this:
~~~
+- main.tex
+ tikz/
| +- ch1/
| +- figure1.tex
| +- figure2.tex
| +- ch5/ % look ahead
| +- figure1.tex
+- titlepage/
| +- titlepage.tex
|- 1.a/
| +- a.tex
| +- a1.tex
| +- a2.tex
|- 2.b/
| +- b.tex
| +- b1.tex
| +- b2.tex
~~~
It's all no big deal: you move intentionally and controlled, like a mountain climber, from save-to-save / safe-to-safe :)
[1]: https://tex.stackexchange.com/questions/759024/best-practice-for-structuring-a-long-thesis-with-input#comment1893595_759024
[2]: https://i.sstatic.net/1AJBsY3L.png
[3]: https://i.sstatic.net/7Kx0nyeK.png
[4]: https://i.sstatic.net/65Qa29wB.png
[5]: https://i.sstatic.net/GW5NiFQE.png