"..."
In your fanfic "..." just looks dumb.
Yes, manga uses it all the time but manga is a visual medium. Fanfic is not. It works in manga because not only do you have dot dot dot, you also (usually) have the character's facial expression, posture, and (often) what they're reacting to in the very same panel as '...'.
When you're writing fanfic, using "..." makes me think you're a.) too dumb or b.) too lazy to tell us how the character reacted. Are they blinking in shock, is their mouth open, are they staring blank-faced ahead, what?
cum
I hate this word. A lot of smut writes who I otherwise enjoy use it and every time the misspelling comes up on my screen, I flinch inside.
First of all, it's just an ugly word.
'Cum' just reminds me too much of 'scum' which makes me think of something I should scrap off my boots after I visit a dive bar. And hey, maybe that's your intention - gritty, dirty, ugly sex between characters who hate each other. To which I say, please show me through setting and action that it's gritty, dirty, ugly sex (and also, vocabulary like fuck, rough, and cunt which, while not exclusively the province of gritty, dirty, ugly sex is definitely on the 'realistic' side of the metaphor/realistic smut scale).
Besides which, whenever I see the word 'cum' I am invariable reminded of fanfics with lines like, "'So, do you like it Ruff, Hermione," said Draco.' Which 1.) is a direct quote from a fanfic I read long, long ago and 2.) I'm very sorry for making you read that.
Lastly, misspelling 'come' to make it clear we're referring to someone's climax is unnecessary.
I am very good at context clues. For example, I can easily tell the difference between 'Sasuke comes into the entryway' and 'Sasuke arrives home in time to see Kakashi pounding himself into Sakura as they fuck against the wall; Kakashi's fingers digging into her hips and face contorting as he comes.'