You could put all the common headers in a property drawer of top-level heading, and all child source code blocks inside that heading will inherit them.

    * Notebook
    :PROPERTIES:
    :header-args:R: :colnames yes
    :END:

    #+begin_src R
    words <- tolower(scan("intro.org", what="", na.strings=c("|",":")))
    t(sort(table(words[nchar(words) > 3]), decreasing=TRUE)[1:10])
    #+end_src

As for making the block delimiter look different, there are various ways of making the text display differently from the actual text. For example, you could use the built-in `prettify-symbol-mode` with this config:

    (setq prettify-symbols-alist
          '(("#+begin_src" . ?)
            ("#+end_src"   . ?―))

You could, for example, use Python which I believe would be clearer than this R mess ;-)

Joking aside though, if src block header (which can have a lot of options set up including specifying environment, tangling other block variable of setting totally separate interpreter version) is huge problem there are plethora of presentation customization.

Emacs allows customizing face and more. Org-modern [1], for example, uses font signatures and fringes for making it less “technical”

[1]: https://github.com/minad/org-modern

I'd expect there will be a lot of new solutions coming soon, because Polymode [0] seemingly solves the hard problem the org-babel struggles against. The immiscible major modes, and all that extra syntax and buffer-switching that org-babel needs to work around that problem.

[0] https://www.masteringemacs.org/article/polymode-multiple-maj...