submit comment by pressing the “ENTER” key in a message

That would mean you cannot have line breaks or paragraphs in the comments because every hit on Enter would send the comments. Are you sure this is what you want?

Mmm. I’d like to try it out indeed …

Would it be possible to have that “enter” key behave as a “submit” button on another input field? I’m thinking of Anti-spam math (https://www.herod.net/dypm/) …

Would this be possible?

For antispam look at plugins like Akismet (it comes with your download) and/or
https://www.ioerror.us/software/bad-behavior/
https://unknowngenius.com/blog/wordpress/spam-karma/

I think breaking an expected behaviour of a key is more counter-productive than useful.
(Actually I have no idea whether it is possible or not… I just find it not so good idea.)