TYPO3 – How can I hide subpages in a specific menu hierarchy for a specific page layout? – PHP

I have the issue that one of the pages in a TYPO3 (11.5.20) environment contains dozens of subpages, which would overly extend the HMENU displayed on the frontend. The suggestion by a colleague was to hide all the subpages in said menu and instead display them on a separate menu embedded on the page itself. This basically means we have to make the subpages hidden for only one specific menu, not for all of them, ruling out the possibility to hide the subpages for navigation by the backend.

To have this functionality not just applying to a single, statically defined page, my idea to approach this would be to register a separate page layout for pages with this requirement and somehow configure HMENU in Typoscript to ignore subpages of that given layout. What I found is the itemArrayProcFunc for processing menu arrays; what I would do is to simply return an empty array with it if said page layout applies. I am encountering two problems though:

• The function I have defined seems not to get called. I am including the script like in the Typoscript snippet below, but even when deliberately throwing an exception inside, there is no feedback on it whatsover by TYPO3, with the menu displaying all pages as usual. Is that method of inclusion possibly outdated? The official TYPO3 doc dictates USER and USER_INT for registering custom functions, but I’m not exactly sure how to make this work together with itemArrayProcFunc.
• Even if the function worked, I’m not sure how to retrieve the layout of the respective parent page, or if it is even possible to retrieve it at all.

I’m assuming there are some major points regarding custom functions within Typoscript I might have missed (to be fair though – TYPO3’s documentation is not exactly transpicuous). Could anyone possibly give me a hint on it? Is there maybe even a more elegant way to hide menus from specific pages?

lib.ts (snippet):

includeLibs.user_menuItemArrayProcFunc = EXT:lraffb_intern/Classes/MenuItemArrayProcFunc.php
lib {   
    ...
    20 = HMENU
    20 {
        stdWrap {
            outerWrap = <nav class="navigation">|</nav>
        }
        entryLevel = 0
        
        1 = TMENU
        1 {
            wrap = <ul>|</ul>
            
            NO = 1
            NO {
                allWrap = <li>|
                wrapItemAndSub = |</li>
                itemArrayProcFunc = user_menuItemArrayProcFunc->process
            }
            
            ACT < .NO
            ACT {
                allWrap = <li class="act">|
            }
        }
        
        2 < .1
        3 < .2
        4 < .3
        5 < .4
    }
    ...
}

MenuItemArrayProcFunc.php:

<?php

class MenuItemArrayProcFunc {
 
        public function process($menuArr, $conf) {
                if (PAGE_LAYOUT == 'pagets__left_no_subpages') // retrieve the page layout here somehow
                        return [];
 
                return $menuArr;
        }
}