Sweet, thanks!

Using this markup I think I was able to achieve what you were asking for:

.site-main .wp-block-group__inner-container {
    display: flex;
    flex-flow: column nowrap;
    height: 100%;
}

.site-main .wp-block-group__inner-container .wp-block-buttons {
    margin-top: auto;
}

But this is not ideal, it's merely a quick and dirty proof of concept. I have a couple of follow up question, since It's a been a while I worked with WordPress:

1. Is there a way to make the columns "stretch" because they currently don't fill their container:

   

   If there is, please remove the height: 100% css markup.

2. Could you add a custom class to the columns? Otherwise the markup will apply to all columns on your site.

3. Could you add a custom class to the button? Otherwise the markup will apply to all buttons on your site.