Pinephone is basically a smarthpone shaped SBC, with much better software situation than rpi, and you can use it as such. I ssh into mine all the time. You can connect anything you like to it via USB hub, incl. the full keyboard and mouse. You can use bluetooth keyboard, and just do normal computing you'd do on your dekstop, etc.
Except for small display and lower performance there's no differnece.
With a Pencil and Procreate, it's really hard to beat for drawing and illustrating. With an external keyboard and some kind of stand writing is a joy, I like it better than on a proper computer because of a ton of little things that help me keep focused and because the device is so portable and doesn't have the laptop form factor with a permanently attached keyboard, with bluetooth periphery it's more like a wireless battery-powered external screen.
Light to medium spreadsheet work is also totally doable, and I've build dozens of slide decks in various apps, with hand-drawn illustrations.
I use a Pi as a mini server, but doing creative work on one, I can't imagine that to be as nice and slick as on the iPad. Last time I tried the PiOS desktop, it definitely wasn't.
For pure pocket sized computing, why not use RPi? It's both much cheaper, more customizable, and it runs Linux. With enough tweaking you can make it run completely headless, plug-and-run mini computer that you can ssh over local network.
I think the biggest problem with the combining idea is that computing in general is about productivity, and phone is about phone stuff.