I saw your tags @jackdaw-sprite and I figured I could offer you some help? You've got a few options to find fics that are actually about your fave who is often sidelined.
My own personal go-to is to use the Search Within Results filter. You can find the filters by clicking/tapping on any tag. If you're on a wide screen like a laptop, they'll be on the right hand side of the screen. If you're on a narrow screen like a phone, you'll need to press the Filters button at the top of the page to open up the menu.
In that menu, scroll down until you're near the bottom. That's where you can find the box labelled Search Within Results. In that box, you can type
summary: Renhardt OR summary: Rendog
When you filter for that result, you'll get 19 fics with that name mentioned in the summary, and if the name is in the summary you've got MUCH higher chances of them being central to the fic.
There's also a way to find fics with a tag in the X position. Fan coders have had this same issue, and so they've created userscripts that you can add to your web browser (like an extension). Here are a few that might be helpful for you to try:
AO3 Actually Relevant Characters - Sorts and highlights works based on how relevant the tagged character seems in the work's tags and summary.
AO3: Prioritise my Faves - Hide work if chosen tags are late in sequence, or if blacklisted tags are early
AO3 Secondary Character & Pairing Filter - Replaces works with placeholder if the tags you want aren't the first few tags, with option to show work.
Editing to add: this version of the post has a syntax error in it. To remove oneshots with a filter, use
-expected_number_of_chapters: 1
izzy806
amandla
ellievsbear
pixelskylines
germansingergirl
gummygunk
