If you loved Fate/stay night: Heaven's Feel III. Spring Song, try Fate/stay night: Heaven's Feel II. Lost Butterfly
A bridge between a film you've already seen and one most people haven't. Here's what they share, and what the second one does that the first one doesn't.
You already loved

Fate/stay night: Heaven's Feel III. Spring Song
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Fate/stay night: Heaven's Feel II. Lost Butterfly
What they share
Both films are directed by Tomonori Sudo, and they both carry the bittersweet, tender mood tags, and they sit in Action / Animation / Drama / Fantasy / Romance territory. If that's the register that drew you to Fate/stay night: Heaven's Feel III. Spring Song, the second film will land in a comparable space — through a different lens.
bittersweettender
What Fate/stay night: Heaven's Feel II. Lost Butterfly is
Apparently someone still needs to save the world. Shirou protects Sakura amid a deadly Holy Grail War. It all gets predictably complicated.