Hey Gregory L. , I'll try to take a stab at answering this:
There's no "preserve text" option on the from-template endpoint, and template generation always remixes/rewrites the content, so it won't carry text forward verbatim (except for cards the template has locked). The good news is you probably don't need templates for this at all: the reason you went that route (the 5,000-character limit) only applies to the additional instructions field.
The main input text field on the standard generations endpoint takes up to 400,000 characters and supports a "preserve" text mode that keeps the supplied text exactly as-is rather than rewriting it. So the reliable path and workaround for verbatim content is: standard generations endpoint > put the full slide-by-slide content in the input text (one entry per page with the multi-page option) > set text mode to "preserve". You can still apply your brand via a theme (just not the template's exact layouts).
But combining preserve text with a template might be a feature request otherwise :/