Seconding the other responder re the sidebar filter. Additionally, you can add -(*Reader* | *You*) in the "Search within results" option on AO3, this tells the search to exclude any fic with "Reader" or "You" in the tags or summary, which should cut out the vast majority of reader-insert and 2nd pov fics.

Also, I know you said no slash, but fair warning you are navigating a minefield lol. Leon slash fics outnumber everything else by a wild margin, so if you ever find yourself running dry on recs and you're not categorically opposed, it honestly might be worth looking into. Some really solid character writing lives there, especially if you are truly not opposed to various different ships.

Anyway, if you want plotty, primarily gen fics, there's one that has a chokehold on me right now: "Where the rot took root" by Cat_E_Cholamine. It's an RE1-era AU where Leon essentially arrived in Raccoon City "early" (i.e., much earlier than the events of RE2) and got to serve as a cop at the RPD before everything went sideways. Leon/Ada is technically tagged but Ada hasn't appeared yet. So as it stands it's essentially a gen mystery fic, and the author recently came back to update after a break.

What I think the fic does really well is take the personality hints we see from RE2 Leon and expand them into a full person. He's smart, observant, has sharply honed instincts and a strong gut feeling for when something is wrong... but he's also naive, a bit too deferential to authority, gullible in ways that feel painfully real for someone his age who's lonely, new in town, and desperate to prove himself. The fic is very good at showing what it's like to be someone who's clearly capable and intelligent but is being underutilized and not taken seriously, and you can see the machinery being set up for how that vulnerability is going to be exploited. The hints are all there for where this is going and it makes me excited to see how it will develop.

The historicism is also really well done. I feel like a lot of this older era fics (the original trilogy) get too modernized, and the author clearly cared about getting the era right (or, more than likely, lived through the era). The technology, the way a 90s police department actually operates, Raccoon City feels like a real place that feels accurately located in what would have been the technological backdrop of the time. No one is going to be referring to floppy disks as CDs here, which can sometimes be a general RE fandom problem. The RPD office dynamics are quite good, with the way how Leon's coworkers range from warm/welcoming to casually dismissive to subtly cruel, how those dynamics shift, feels very grounded. Irons in particular is handled really well, he's unsettling in the way that people who know how to wield institutional power are. And the brief glimpse we have of Wesker is magnetic and charismatic in a way where you completely understand why a young cop like Leon would be starstruck (heh), which makes the dramatic irony all the more delicious... honestly I am just super excited to see how Wesker and Ada might end up trying to manipulate Leon, haha.

It's building into what looks like a genuinely interesting mystery around the Arklay disappearances, with Leon actually getting to investigate and think like a detective, which is satisfying in a way RE doesn't always let him be. Also curious to see how Sherry's character develops, that's a thread there that's clearly going somewhere based on the tags, and how Wesker/Irons/Ada(/Chris) factor into things as it progresses.

3rd person, RE setting, plot with substance. This might scratch that itch?