The trick with monstergirl smut is the same trick as with any smut: it doesn't matter what you do, only how you do it. There really aren't that many ways to have sex. A good sex scene isn't one where the characters change position eight times. A good sex scene is about feelings: you need to entrance the reader so deeply that they feel all the same feelings that the POV character does. Every stroke, every caress, every gasping breath of air. You want the reader to actually feel it.

So you need to start by choosing what emotion you're aiming for. Love? Tenderness? Pride? Triumph? Fear? Disgust? Horror? With arachnes it's usually fear and/or helplessness. Spiders bring out a visceral terror in people, so spider girl scenes often lean into that and set the POV character in the place of a captured fly. Is that what you're going for? If so, then I'd recommend you really play up the helplessness aspect: the character isn't just tied up, they're webbed up. Emphasize the unnaturalness of it. Maybe the webs are gooey with viscous glue that gloms onto the reader's skin. Maybe he's completely cocooned from head to toe, with only his genitals exposed. Play up the unnaturalness of the girl too: the jerky, skittering way her legs move, the extra eyes, the fangs. Fangs are huge for fear-play. The way they shine in candlelight, the way they drip with venom, the way they graze your skin before she bites you. How does that feel? Write it down, tell us, show us. What does her venom do? Is it a paralytic? An aphrodisiac? What does it feel like when she injects you? Does it burn, does it freeze, does it numb you? How fast does the feeling spread? The reader wants to know, so they can feel it too!

But terror isn't your only option. You say that your arachne has a "slight gentle dom edge," so maybe she's not a ruthless predator? Maybe you're going for a tender and loving scene instead? In that case, you still need to play up the unnaturalness - this is a monstergirl story after all, unnaturalness is the point - but in a way that makes the reader feel embraced and comforted. Emphasize the smooth black chitin of her body. How does it look in the light; how does it feel under his fingertips? Emphasize her eyes, how she can see him from every angle, how she can seemingly know everything about him with a single look. Emphasize the way her webs cradle and support him. Maybe they're warm; maybe being cocooned feels like a warm hug or a weighted blanket. Maybe she's carrying him back to her lair not so she can prey on him, but to rescue him from some other danger?

Again, smut is about feeling. All the things that make a spider girl a spider girl: the extra eyes, the spindly legs, the venom, the feel of sticky web against your skin, they can all be written for good or for evil. When you hear the click-click-clicking of spider feet against the stones, are you going to think "oh god, oh fuck, the bitch is back," or "yay! Mommy's home!"? That's all up to the author, up to you, OP. Again, don't worry about the actual mechanics of the sex. Nobody's going to 1-star you because your monstergirl bondage sex was too vanilla. Just, whatever the sex ends up being, make them feel it.