Non-suggestive is a very difficult bar to meet given the subject matter. Pretty much everything in fantasy-lite has fanservice to make up for lack of originality. Outside of fantasyland I imagine there are some good horror stories directly in line, but I don't want to take the time to dive into those.
But, in magic worlds there are at least a couple suggestive titles that feel adjacent to this:
https://mangadex.org/title/1aa09c5e-d6cd-4662-93f2-8c64331b6234/monster-no-goshujin-sama: I haven't read for a couple years, but IIRC the MC gains control over a slime, who in turn gains control over (or maybe just dupes?) a deceased classmate(s?). She's not the POV character, though, so only a parallel concept to this story. Not for kiddos on account of, well, check the tags.
https://mangadex.org/title/9f86feea-b572-4fba-9cf5-e328a3c67614/plant-monster-girl-diary: pretty cute, only vaguely suggestive (there is a bear that licks the honey from her hair, her hair), but the crux is the MC gets eaten and then hybridized with a plant monster. The hybridization is the important part, so not really like "monster possessing a human" , or a "human possessing a monster", because her new identity is a fusion of both.
Narratively, any sort of possession story would be tricky to pull off since you would need the possessor to be nearly human mentally in order to be an effective MC. Even with this story, the slime's original sentience really strains at the suspension of disbelief. H-works don't have that limitation though, because plot! is just window dressing at best.
It's not that it's a hard standard to meet it's that no one wants to do it. And why would they? Fan service and sex sells and slime possession is
insanely low hanging fruit for that, hence the large volume of ecchi and hentai about it, pioneered in large part by Date. And, well, fine. Artists know the market. Go nuts. That is
not my issue. The person I quoted would rather upthumb you than address the fact that they were recommending extreme hentai, without saying that it's even hentai at all, in the forum of a series that doesn't even have the suggestive tag, so far (suggesting it might be aimed at a younger audience) and say that it's "better". Sending multiple users, some of whom might be underage, to visit exhentai (sad panda) to find it thinking it's something like
Slime Saint but better which it
totally isn't. Why would anyone do that?? Recommendations are fine (I like those), but it would be nice for someone to address that doing what they did is a pretty f'd up thing to do.
And they appear to be running and hiding behind your comment as if it's some kind of approval.
First suggestion is, as you said, not for minors, but at least you actually freaking mentioned that in your post, unlike the other poster. I definitely do
not appreciate SV so this one is a hard pass even if it shows up only once. My experience with SV in animé, manga and visual novels has often been little more than a cheap plot device to motivate a character (usually the male lead) into action (unless said character is actually the one doing it. . . Fun. . . -_-). It's so seldomly treated seriously and with respect that I see the tag and just run immediately. I don't need that shit. If others do, fine. I'm not saying it shouldn't exist and that people shouldn't consume it and enjoy it and I've gotten relatively good at avoiding it myself. The tags on MangaDex have been incredibly useful for that (for the most part. . .). But users shouldn't be recommending hentai in the forum of a series that looks like it might be aimed at a younger audience. That should just be fing common sense.
Second suggestion sounds much better. Suggestive tags don't throw me off because that could mean anything from 1 scene to every single f'ing chapter (which is what I don't care for, at that point it's basically SH/SA). So I might actually give that one a go. I'm not into vore, though, but it's not tagged as gore so it can't be that bad (right?). I can imagine the scene where she gets consumed and turned into an alraune to potentially be suggestive. Again, honey or sap, it's often very similar to slime, so insanely low hanging fruit there. Even Mira, a yuri hentai artist (there's your content warning, 18+) with
mostly vanilla content has gone there more than once. It does sound more like fusion to me than actual possession but I guess it would depend on how it's presented. I wouldn't consider
3x3 Eyes,
Elfen Lied (content warning, SV and gore for both) or
SHUFFLE! (another warning, very suggestive, visual novel is hentai) to be possession stories. They're just 2 minds sharing the same body, mostly willingly and with consent. I guess I associate possession more with an actual takeover of the body (even if the host is dead,
Slime Saint) or a struggle between the 2 minds with each other. A complete, or attempt at a complete, takeover of the host body.
Second one is most like
Slime Saint for sure, even if not actual possession. I'm not desperate for clean possession content I was just pointing out how insanely rare it is to see and wouldn't mind seeing more.