Couldn't I use the internal PCIe/SATA for the new graphics card, and an external hard drive for the boot? Alternatively, could I replace the Geforce GTX 1050 with a more powerful card?
The 1050 is almost certainly just built onto the motherboard.
Not changeable.
An external for the OS drive?
Even if you get all the trials and tribulations to get that running (non-trivial)...you're exchanging one slowness for another.
Running a Windows OS from an external is vastly slower than a regular internal drive.
You are MUCH better served by buying a whole new system that actually does what you need.
Sell this fashion piece AIO, and get a real PC or laptop.
Alternatively...
Do you
really need actual Photoshop?
It is a resource heavy application.
And only really needed in a production environment. There are loads of other graphics applications that do 98% of what PS does, without all the Adobe overhead.
And if you DO really need real PS, a functional PC is just another business expense.