How can I create my desired template name suggestion?
If you want to have more meaningful template suggestions, why don't you use better region names and block ids?

Region names are up to your theme, so instead of footer_1, use footer_something, and block configuration names are also up to how you configure/name your blocks.

You could use PHP to filter those things out, that's nothing really drupal specific, but that's likely increasingly complicated as you will have blocks and regions without those number suffixes, so you need to be caruful in not cutting those off.

Also, you could use suggestions based on the block plugin ID, not configuration ID, then placing the same menu multiple times will give you the same template suggestion. 

Edit: If you really want to get rid of those kind of suffixes, then you could for example use trim:

rtrim('footer_1', '123_');


You can put whatever characters in there that you want, and it will remove them all from the right side of the string (there is also ltrim for left and just trim for both sides). Note that this will also remove a _ at the end if you have that.