Affinity Designer for iPad was just released and it sets the bar for an iPad app (even out-doing Affinity Photo for iPad).
It might be worth spending the 13 bucks to check out its extensive use of gestures. I think that you might find inspiration for doing some similar things for Agenda for iPad.
A couple that come to mind that might apply to Agenda:
Two finger tap for undo. Three finger tap for redo. And you can drag on most of the icons on the right side (what they call “studios’) to quickly change settings. For example, dragging on the Text Studio icon allows you to quickly change the text size without opening up the Studio. The Stroke studio allows you to do the same with stroke size.