I’ve really appreciated the ability to add sketches into my notes using my iPad. Recently I needed to upgrade to a MacBook so I’m beginning to experiment with stylus and software options to use my trackpad as a mini tablet. Of course it’s nowhere near the functionality of an iPad, but for the types of quick sketches I need to do (whiteboards on Zoom, Agenda notes) it will do the trick.
Currently on Mac, however, the only way to insert a sketch/drawing is via a companion iPhone or IPad. I get the idea and it works well (especially if you want to replicate the drawing tool UI from iOS), but finding a way to include the trackpad as an option would eliminate the need to toggle between devices and (in my case, sans iPad) give me a larger and more intuitive surface.
There’s probably way more that would go into this sort of feature that I can’t anticipate, but if there’s a simpler way to make this possible that I’m not seeing I’d love to hear about it!
The problem is more that Agenda is a note taking app, focussed mostly on text, and any drawing tool we make would probably have to be pretty simple. We couldn’t even duplicate what we do on iOS, because that is from a framework provided by Apple, with all their resources.
What I would suggest is finding another app on macOS that works well for drawing for you. You probably even have one.
Agenda 14, which is coming soon, has a nice feature: you can edit attached files in other apps, and the changes stay in Agenda. So what you could do is, for example, drag a Pixelmator document into Agenda, then double click it to open, edit, and save changes back to Agenda. And they sync to all devices etc. This should be a good workflow that allows you to have a great drawing tool, and still keep things in Agenda notes.
I’m definitely not giving up on Agenda! 
The Continuity sketch via an iPhone works well enough for my purposes, was just hoping for a fully integrated possibility (even if, as you said, it was something very simple). The updates for 14 sound really intriguing and may work just as well, and in a split screen workflow may accomplish what I’m looking for.
Thanks for your thoughts!