Well, those ARE your fatal errors. They all say that there are missing data in your system. I will go over them a little, but, not sure if I can fix it for you without knowing more about the entire site.
First two errors are about classes. A class is basically a function, but, it is an OBJECT function, not just a simple list of commands. They both say that the class_name variable is missing on line 32 and 44 of the functions.php file. Therefore, you will need to look at the functions.php file and those two lines to see why that variable is missing. Most likely a previous command above those lines is not creating the variable correctly.
The third error is similar, but, harder to explain. An undefined-index means that you are accessing an array and the array index passed to it does not exist inside array. So a simple array like this:
$colors=(1=>“black”, 2=>“white”,4=>“pink”); would have 3 entries. A numeric index or key and a text value. If you try to echo it like this: echo $color; The index does not exist and it throws an undefined index error. I think that is easy to understand. Now, in your line 773 of the image code which is named autoptimizeimages.php, the code is using autoptimize_imgopt_number_field as an index into the array. You need to look at that line 773 and see where that index is being created. Make sure the spelling of it is correct as it is an oddly named index. Either it is spelled wrong or it is not being created before it is used. Again, we would need to see more of the code to assist you in fixing that error.
At least we now know where the errors are. So, you can review these pages and see what is wrong. I suspect that there might be a code where those three indexes are created that is deprecated. What that means is that you might be using a command that is no longer in your version 7.4.
Also, you might want to reinstall WordPress. I have seen in the past that when you install a newer version of PHP, WP gets messed up and if you reinstall it, it fixes pointers which might be an issue.
I would start by looking where those three indexes are created and attempt to fix them that way first.