Apache IF condition based on custom HTTP header

I have a RewriteRule directive in Apache that I would like to get applied only if a custom HTTP header is present.

What I have tried:

<If "%{HTTP:X-MyCustomHeader} in { 'present' }">
RewriteRule ^/$ /present.html [NC,R=301,L]
</If>
<Else>
RewriteRule ^/$ /absent.html [NC,R=301,L]
</Else>

This does not work at all, neither RewriteRule gets applied.

Alternative that I have tried:

<If "-n req_novary('X-MyCustomHeader')">

Does not work either.

I am testing with curl like

curl -is http://example.com/ -H X-MyCustomHeader:present

To validate that the header is present and seen, I tried using an env variable:

SetEnvIf  X-MyCustomHeader ^(present)$ MY_CUSTOM_VAR=$1
Header always set X-MyCustomHeader "### FOUND ###" env=MY_CUSTOM_VAR

This works as expected, and I get the FOUND in the response.

But again when I try to use the env variable in the IF statement, it will not get triggered, e.g.

<If "-n env('MY_CUSTOM_VAR')">

How can I do this with IF? And yes, I know there is a RewriteCond condition to check environment variables, but I need to use other directives as well, so I would like to switch them in an IF block.

According to the docs: https://httpd.apache.org/docs/trunk/en/expr.html#functions

When environment variables are looked up within an condition, it's important to consider how extremely early in request processing that this resolution occurs. As a guideline, any directive defined outside of virtual host context (directory, location, htaccess) is not likely to have yet had a chance to execute. SetEnvIf in virtual host scope is one directive that runs prior to this resolution.

So according to the docs, SetEnvIf variables should work, but they don't?