Thank you! Romance isn't a priority when I'm searching for novels, either -- although I do like some of them, I'm generally ambivalent about the genre itself.

To be honest, I'd really like to find something similar to ITITMB too, hahaha...especially if it's also mystery/horror/etc. Unfortunately, I haven't really got anything, and most of the unlimited flow/quick transmigration novels I've read are BL. Still, here are some recs that fit a few (if not all) of the conditions you're looking for, perhaps? They're not that similar to ITITMB overall, merely on a few points here and there, but nonetheless. Sorry I couldn't be of more help.

"Low IQ Supporting Female, Waiting for Death Online" has some decently emotional scenes, some twists & developments with the overarching plot, and while there's some one-sided affection from the various male characters towards the FL, the FL never develops feelings and consistently makes an effort to be a villain. The "real ML" (if any) shows up at the last chapter for a couple of paragraphs, and that's it. However, it's not horror or thriller, and the character development isn't nearly as good.

"I Was Admitted To The Villain School" has some very neat twists towards the middle-end, although most of the novel is less serious and more comedic. There's some romance, but I think comedy takes priority. 

And two novels I haven't read yet: "The Chronicle of the Oriole Island", which is a rather short, apparently well-written mystery that is allegedly romance-free, and "A Hint of Spring" is another mystery/crime novel that's apparently low on romance.

@evileye23 thanks for the recommendations! I just finished reading apocalypse lord and came back to the list to read more from your recommendation list 

I already read the villain school one but I’ll check out the others. Tbh I was really surprised when I found ITITMB, that was a real hidden gem. I think the title doesn’t do the work justice , it sounded like a second rate novel title  but it was definitely a masterpiece *chef’s kiss*

Not to underestimate your undoubtedly profound degree of literacy, but there's an "all pairings" note in the title of this list, as well as the "mainly BL/het" note in the description. 

One would hope that you, a literate person on a site for aggregating novel information, would know to read a list's title and basic description before clicking on it. 

Of course, I'll be assuming the best of you. As such, thank you for agreeing with my choices. I hope you enjoy my novel recommendations, especially the BL ones, which seem like your favourites, since your comment is so focused on them (despite the variety of other novels on my list).

Since you love BL so much, if you're looking for more BL recommendations, feel free to let me know.

Also, if you're looking for "het male MC x solo FL", there are already specifically harem-free MC lists. The ones I've read tend to not have any substantial romance, but romance isn't nearly my favourite genre, and seeing as this is a list of my favourites, it's not all that personally relevant. 

As for actually romantic ones, idk, good luck, gotta be somewhere out there. Unfortunately, A Harem Of Cardboard Beauties is a lot more popular with straight male MCs compared to 1 on 1 romance and a FL with personality. 

Edit: come to think of it, here's a non-BL rec. The most recent work I've read with decent writing, engaging plot, and a male primary MC that ends up with a girlfriend at the end is Snow Crash. 

(No harems! Not BL! Girlfriend endgame for our charming male MC! Perfect for the picky reader ignoring others' list criteria!) 

(The next sexy romantic novel with a male MC on my to-read list is Dream of the Red Chamber. Not BL! I've even heard that it's full of fascinating tales of love, life, and romance.)