[SalesForce] Pass variable from Apex to Classic Email Template with Replace

I have a classic email template which includes a {!TODAY() + x} function. I want to dynamically steer this x via APEX.

Therefore I use .replace() function in APEX which will look like following:
{!TODAY() + xPLUSDAYS} in Email Template and emailHTMLBody = emailHTMLBody.replace('xPLUSDAYS', days); in APEX where days are dynamically declared, e.g.: String days = '14';

Unfortunately when I try it out, this will lead to a blank area where the date should be. I already tried it directly with e.g. {!TODAY() + 14} and it worked totally fine (showed e.g. 03/24/2020, when today is 03/10/2020).

I went ahead and changed classic email template to {xTODAYPLUSDAYS}, APEX to emailHTMLBody = emailHTMLBody.replace('{xTODAYPLUSDAYS}', days); and declare days like String days = '{!TODAY() +14}'; so that the whole TODAY() function string is changed in a whole.

Unfortunately this will lead to the email actually showing {!TODAY() +14} as Text instead of e.g. 03/24/2020.

I also tried variations of the replace string, with ! or without, with {} or without, but the basic result always remains one of the two mentioned above (nothing shown or faulty code string shown).

I render the email as follows: Messaging.SingleEmailMessage email = Messaging.renderStoredEmailTemplate(templateId, u.Id, u.Id);

Then I go ahead and alter the html body as explained:

String htmlBody = email.getHtmlBody();
htmlBody = htmlBody.replace('{xTODAYPLUSDAYS}', days);
email.setHtmlBody(htmlBody);

Finally I go ahead and send that thing:

Messaging.sendEmail(email, false);

(I actually do all these things within loops for multiple recipients with an allEmail List and a user List, but that shouldn't make a difference)

I already had a look at those "replace merge fields" articles, but I figure this particular issue is quite specific in regard of altering a function in html.

Is there any way to alter a function within the HTML to make e.g. this today+x date flexible? Thanks.