The appeal of Ichigo and Yoruichi's dynamic is that crackle of power and history they share, which makes slow-burn a perfect fit. You need fics that let that tension simmer, not just jump to the obvious. I'd steer you towards 'Thunder in the Soul' on AO3, which is this long, patient story set after the war. It has them rebuilding Karakura Town together, with Yoruichi dealing with the fallout of the Soul Society's politics and Ichigo adjusting to a quieter life. Their bond deepens through shared silences and protecting the same people, not grand declarations.
Another one that gets overlooked is 'Feline Grace, Substitute's Pace'. It's a time-travel premise where a post-TYBW Ichigo ends up back in the Soul Society arc, but the focus is less on fixing canon and more on his changed, more weary perspective clashing with Yoruichi's playful but deeply observant nature. The romance is so secondary to the character study for like 30 chapters that when a shift happens, it feels completely earned. Avoid anything tagged 'smut' or 'fluff' too early; you want the 'angst', 'emotional bonding', and 'mutual pining' tags to be your guides.
Honestly, the tag system on AO3 is your friend here. Filter for Ichigo/Yoruichi, sort by kudos, and then just start skimming summaries for words like 'gradual', 'post-canon', or 'alternate first meeting'. That's how I found 'Static Cling', which is this wonderful modern AU where Ichigo's a mechanic and Yoruichi's a journalist investigating his shop. It still captures her elusive energy and his blunt honesty perfectly, just in a different setting.