Hi, in the Console I can see a few warnings.
You could right-click on the page, then select Inspect Element. Then click the Console-tab for the panel with errors and warning.
I see you use this plugin:
Which is unmaintained for 5 years. You want to look for a replacement.
I also see some JS warning on line 625 of your main index.html page, but I don’t know which plugin or theme it is coming from.
If you would fix these 2 problems, you should be able to disable the jQuery Migrate again and be able to update to WP 5.6 next winter.