you have plenty of options

Option one: You can replace your current cassette with one that has a larger cogs, you can probably go as big as 30 (28 for sure). This relatively simple change would lower your lowest gear from 45 to 35 gear inches.

You could also replace your crankset with a compact crank, like the gran cru crank you linked to. A compact crank is a double crank that has smaller chainrings than a conventional crank (usually around 48/34). You can probably install that crank using your current front and rear derailer. That particular crank would give you a low gear of 30 gear inches with your current cassette, or 26 if you swap out your cassette as well. If you do this, your derailer might have trouble with the chain slack if you use your small/small combination, but as long as you're cognisant of this risk, you should be allright.

If you want true stump pulling gears, you're going to need a triple. This will require not only a new crankset, but also a new front and rear derailer. It would be an expensive endeavour, but you could get immensely low gears.