I do pretty much all my Glowforge business on iPad. (Even GF’s web UI, despite it being a bit jankier than on desktop.)
Vectornator is free, and pretty decent. The learning curve seems a bit less steep than some of the more serious commercial vector apps, so my kids (age 5 and 9) have been learning it for their own first steps into GF design. And I often prefer it for “quick and dirty” work or stuff that doesn’t need a more serious feature set.
Affinity Designer is the most capable app, but with great power comes great inscrutability. Be prepared to spend more time digging through the help to figure out what unlabeled buttons mean. Thanks to the COVID crisis it’s on sale for half off ($20 US), and there are deals on the desktop Affinity apps if that’s your thing.
Supposedly there’s a “lite” or “mobile companion to desktop “ version of Adobe Illustrator, too, but I haven’t tried it.
Also, the free “iWork” apps from Apple (Pages, Numbers, Keynote) aren’t to be overlooked. They’re a lot less capable than a dedicated illustration app in most ways, but for a few tasks they can be easier. And they have some niche features that the illustration apps don’t: for example, I used Pages for most of the layout in my recent Japanese blocks project because of the vertical text support, then exported to PDF and tweaked it for GF output in Affinity Designer.