Keep the same paragraph style. Apply a specific character style. Or, if you want both heading and TOC entry upper-case, type upper-case. IMO, the simplest to do. Don’t ask too much to styles. Styles must not obscure both structure and meaning.
I see I was inaccurate. Sorry.
It is about back matter. Toc is in frontal part of document, not in back matter.
Back matter will use one single level of outlining/headings.
Headings in back-matter are semi-numbered, it means “numbered”, like chapters, but by Capital letters instead of digits.
TOC will cover back matter.
Furthermore the back matter gets own page numbering, but only symbols used for numbering is differ than in remaining document parts. Roman digits in back matter for page numbering. I believe this will require dedicated page style. I believe heading style used in back matter will need to refer to page style dedicated for back matter.
This is advanced usage. Creating twosequences of differently numbered outlines in a single document requires mastership in paragraph styles (granted), outline specification and list styles.
Keep this exercise for later once you have tamed the main part of your book.