I assume you mean 'Write access' field in the form.

make the pages in the pages plugin always "private"

You need to create an event in your custom plugin:

elgg-plugin.php

'events' => [
        'access:collections:write' => [
            'user' => [
                '\MyPlugin\Pages\Views::onlyAccessPrivate' => [],
            ],
        ],
],

MyPlugin\Pages\Views.php

<?php

namespace MyPlugin\Pages;


class Views {

    public static function onlyAccessPrivate(\Elgg\Event $event) {
        
        $input_params = $event->getParam('input_params');
        $return_value = $event->getValue();
        
        if (empty($input_params)) {
            return;
        }
        
        if (elgg_extract('entity_subtype', $input_params) !== 'page') {
            return;
        }
    
        if (elgg_extract('purpose', $input_params) !== 'write') {
            return;
        }
        
        unset($return_value[ACCESS_PUBLIC]);
        unset($return_value[3]);
        unset($return_value[ACCESS_LOGGED_IN]);
        
        return $return_value;
    }
}

remove the access dropdown list 

You need to override \mod\pages\views\default\forms\pages\edit.php in your custom plugin 

In \mod\my_plugin\views\default\forms\pages\edit.php edit this code:

switch ($name) {
        case 'access_id':
        case 'write_access_id':

On this:

switch ($name) {
        case 'access_id':

Nevermind!  Figured out the correct way to do it seems to be to create a new mod that overrides the css.  If it's any use to anyone else I just create this structure.

tdntheme/
├── composer.json         # Plugin metadata
├── elgg-plugin.php       # Plugin configuration  
├── README.md
└── views/
    └── default/
        └── tdntheme/
            └── css/
                └── custom.css   # Your CSS overrides

And added this to the custom.css:

/* ==========================================================================
   Walled Garden (Login Page) Mobile Fixes
   ========================================================================== */

@media (max-width: 768px) {
    /* Fix: Login page content getting cut off on mobile */
    .elgg-page-walled-garden > .elgg-inner {
        min-width: auto !important;
        max-width: 100% !important;
        width: 100% !important;
        padding: 0 1rem;
        box-sizing: border-box;
    }

    /* Make the heading wrap properly */
    .elgg-heading-walled-garden {
        font-size: 1.5rem !important;
        line-height: 1.3 !important;
        word-wrap: break-word;
        overflow-wrap: break-word;
    }

    /* Ensure page body doesn't overflow */
    .elgg-page-walled-garden > .elgg-inner > .elgg-page-body {
        padding: 0.5rem !important;
    }
}

/*
=== Didn't need this but if the site title was longer... ===
@media (max-width: 480px) {
    .elgg-heading-walled-garden {
        font-size: 1.2rem !important;
    }
}
*/

/* ==========================================================================
   Mobile Responsive Fixes (Logged-in pages)
   ========================================================================== */

@media (max-width: 768px) {
    /* Fix: Site title getting cut off / center-justified on mobile */
    .elgg-page-header .elgg-heading-site,
    .elgg-page-topbar .elgg-heading-site,
    .elgg-heading-site {
        text-align: left;
        overflow: visible;
        white-space: normal;
        word-wrap: break-word;
    }

    /* Ensure header content doesn't overflow */
    .elgg-page-header,
    .elgg-page-topbar {
        padding-left: 10px;
        padding-right: 10px;
    }

    /* Make site title more readable on small screens */
    .elgg-heading-site a {
        font-size: 1.2em;
        line-height: 1.3;
    }
}

@media (max-width: 480px) {
    /* Extra small devices */
    .elgg-heading-site a {
        font-size: 1em;
    }
}

/* ==========================================================================
   Group Page / Owner Block - Mobile Fixes
   ========================================================================== */

@media (max-width: 768px) {
    /* Stack image and description vertically on mobile */
    .elgg-image-block.groups-profile {
        flex-direction: column !important;
        align-items: center;
    }

    /* Limit the group icon size on mobile */
    .elgg-image-block.groups-profile > .elgg-image,
    .elgg-image-block.groups-profile img {
        max-width: 150px !important;
        height: auto !important;
    }

    /* Give the description full width when stacked */
    .elgg-image-block.groups-profile > .elgg-body,
    .elgg-image-block.groups-profile .elgg-profile-fields {
        width: 100%;
        text-align: left;
        margin-top: 1rem;
    }
}

There's probably some work to do with Pages too, but I ran out of time.  I'll find some time this week to do those, find the right-hand-side Pages menu that vanished somewhere along the line, and fix my personal bug-bear: members lists and counts including banned users literally everywhere...

I didn't have time to respond; you were quicker.

This is a problem with the current Elgg layout.

We often have to fix it using styles in custom plugins, but there's no single solution since 3rd-party plugins can also have their own styles, etc.

I'm glad you fixed the issues in your custom plugin!

Thanks. I will do that.

I figured out what happens in poll: it does not load chart.js

At first glance it looks like a javascript loading issue. You already mentioned the chart.js from Poll not loading.

Check the browser console (F12) to see if there are any more javascript issues.

Try to isolate the issue to a single plugin (by disabling plugins one by one) and hopefully you can find the origin of the issue.

We have several website where all these plugins are active at the same time and don't have an issue. If you find it, please contact us.

Thank you, Jerome.

I will follow your advice and see what I can find.

I fixed the poll plugin functionality with a small edit to Bootstrap and putting the chart.js files inside the plugin, but if there is a better way, I will try that. 

I posted the event manager issue to github.