Increment a progress bar in a windows10 batch file

I've excerpted a part of the batch file that I'm using. I was experimenting with making progress bars, instead of numerical counts output by timeout.

My question is, how can I dynamically modify the string in a loop, rather than having to retype every step of the string?

@echo off

setlocal enabledelayedexpansion

:: This is to set variables that represent backspace and carriage return.   
:: I begin strings with these, instead of ending strings with only CR,
:: so that the cursor isn't flashing under the first character of the     
:: progress bar. Beginning each string with a space, and ending each 
:: with !CR! only is another possible workaround.

for /f %%a in ('copy /Z "%~dpf0" nul') do set "CR=%%a"
for /f %%a in ('"prompt $H&for %%b in (0) do rem"') do set "BS=%%a"

<nul set /p"=        [          ]" & Timeout /t 1 >nul
<nul set /p"=!BS!!CR![■         ]" & Timeout /t 1 >nul
<nul set /p"=!BS!!CR![■■        ]" & Timeout /t 1 >nul
<nul set /p"=!BS!!CR![■■■       ]" & Timeout /t 1 >nul
<nul set /p"=!BS!!CR![■■■■      ]" & Timeout /t 1 >nul
<nul set /p"=!BS!!CR![■■■■■     ]" & Timeout /t 1 >nul
<nul set /p"=!BS!!CR![■■■■■■    ]" & Timeout /t 1 >nul
<nul set /p"=!BS!!CR![■■■■■■■   ]" & Timeout /t 1 >nul
<nul set /p"=!BS!!CR![■■■■■■■■  ]" & Timeout /t 1 >nul
<nul set /p"=!BS!!CR![■■■■■■■■■ ]" & Timeout /t 1 >nul
<nul set /p"=!BS!!CR![■■■■■■■■■■]" & Timeout /t 1 >nul

As you can see, I currently type eleven commands (well, copy/paste then modify each line) to create the progress of a progress bar.

I would prefer to have a loop that adjusts the width of the bar to the number of times it will loop, then replaces the leftmost space with a block each loop. It would let me re-use this for a wait of any length, or even to show the progress of batch processes that I don't want to flood the screen with, without needing extensive modifications each time.

Unfortunately, I'm not honestly very knowledgeable about batch; I know enough to hack things together with some Google-Fu, but my search terms are not giving me good results with this, and I am not sure that what I'm trying to do is feasible. Any help, even just a starting point, would be appreciated.

I'm not sure I understand your question, but here's a suggestion based on substring and for /l loop:

@echo off

>nul chcp 65001
set "_spc=          "
set "_bar=■■■■■■■■■■"

setlocal enabledelayedexpansion

for /f %%a in ('copy /Z "%~dpf0" nul')do for /f skip^=4 %%b in ('echo;prompt;$H^|cmd')do set "BS=%%~b" & set "CR=%%a"
for /l %%L in (1 1 10)do <con: set /p "'= !CR!!BS!!CR![!_bar:~0,%%~L!!BS!!_spc:~%%~L!] " <nul & >nul timeout.exe /t 1

endlocal & goto :eof

Or some like:

enter image description here

@echo off 

>nul chcp 65001 
setlocal enabledelayedexpansion

set "_spc=          "
set "_bar=■■■■■■■■■■" 
set "_msg= Progress bar loop."
<con: color 0A & mode 70,4 & echo\ 

for /f %%a in ('copy/Z "%~dpf0" nul')do for /f skip^=4 %%b in ('echo;prompt;$H^|cmd')do set "BS=%%b" & set "CR=%%a"
for /l %%L in (1 1 10)do <con: set/p "'=!CR!!BS!!CR![!_bar:~0,%%~L!!BS!!_spc:~%%L!]!_msg!"<nul & >nul timeout.exe 1

endlocal & color & goto :eof