Hi Lisa,
We have to do it one at a time, and the first thing to do is fixing the issue we’re seeing. I only recommended fixing the URL configuration of your site and again, it is not related to migration. That is because it’s still configured to the old URL, and it’s even related to the theme or builder related, but it just affecting them as it still loads the preview from old URL.
And even if you install a plugin or another theme but still relies to the correct URL, then it will still the same if you have misconfigured URL. I like to take screenshots so I could show you what I’m really referring but looks like the login credentials are now changed.
I’m just requesting a URL fix as our first step, and it’s easy to do , please check this https://wordpress.org/plugins/better-search-replace/, it’s displayed on screenshot and no complex steps.
This is also to help you troubleshoot your site and for future issues. And I love to find out the root cause of this issue, but currently, I can’t recommend anything that will solve it in instant since my investigation is just based on the visible errors that I could see. Fixing the URLs is just one of many possible steps, and it’s the first.
Thanks!