Help topic "Key Names" enumerates what key combinations can be mapped to actions in TCC. No more than one of the prefixes Alt-, Ctrl- and Shift- can be used regardless of Windows delivering them or not to TCC. The BkSp key can be prefixed only with Alt- or with Ctrl-, but not with Shift-. You cannot use the Shift-BkSp combination for anything, but you can use Alt-BkSp or Ctrl-BkSp.
Since you need to train your fingers to a different combination than what you are accustomed to, why not Ctrl-L, the default one?
Of course, you are free to create a key handler plugin, which would intercept the ESC-BkSp sequence, and send ctrl-L to TCC instead... But this would interfere with the use of the ESC key during keyboard command entry: erase the whole line. You could, of course, detect the ESC key and wait a specified time before deciding how much to erase - the whole line, or just the word on left.
The other point is the way key mapping works in TCC. Read Help topic "Key Mapping Directives". There are four separate key mappings; any key may be mapped to actions in each that are entirely unrelated. To make a key which is mapped to an action in one of the default mappings available for a different action, it needs to be unmapped first by the relevant normal key directive: NormalKey, NormalEditKey, NormalListKey, or NormalPopupKey. If you don't expect to use the defaullt key for some other purpose, nmapping it is unnecessary.