How to migrate custom buttons and dialogs to TinyMCE 5

I have a 3rd party Bootstrap plugin (probably EOL) that adds buttons to the toolbar which in turn open a dialog where Bootstrap elements can be selected to add to the content.

It uses the following code:

var insertBtn = tinymce.ui.Factory.create({
    type: elType,
    classes: elClass,
    text: bsText['button'],
    icon: 'icon-btn',
    name: 'insertBtnBtn',
    tooltip: bsTip['button'],
    onclick: function() {
        showDialog('bootstrap-btn.php', 'Add button', 580, 'bsBtn');
    }
});
bsItems.push(insertBtn);

But it says Factory is undefined, or create is a function that doesn't exist. What can I use to make this work, showing the button as well as the dialog when clicked? I have updated the following code to view bsItems:

editor.ui.registry.addButton('bootstrap', {
    type: 'buttongroup',
    classes: 'bs-btn',
    items: bsItems
});

I tried several other possibilities to find the create function:

editor.ui.registry.create()
editor.ui.Factory.create()
editor.ui.create()
tinymce.ui.registry.create()
tinymce.ui.Factory.create()
tinymce.ui.create()

to no avail