Key features:

• PDF annotation
• S-pen/stylus support
• Sync (preferably NOT just via Nextcloud…looking at you, Saber)
• Handwriting recognition is a plus, but not required

As mention above, Saber fits the criteria well, but I don't use Nextcloud and that is the only sync available in that app (unless something has changed in the last couple of weeks). I have a Galaxy phone and tablet that both support S-pen and Samsung Notes is excellent, and free, but they're not exactly privacy concious, so I'm looking elsewhere. I had considered the Excalidraw plugin for Obsidian, but I'm not sure if that works on the Android version of the app.