Shimano Tiagra - Install smaller front chainring
The Trek Checkpoint ALR4 has a Tiagra 4720 series groupset. The FC-4700 crank as a bolt-center-diameter of 110mm which will not support a small ring below 34 teeth, so yes you are looking at a whole new crank to get smaller rings.

The RD-4700 derailleur will not work directly with an 11-42 cassette, although 11-36 will probably work OK.

You should be able to keep the front derailleur - 48-32 or 46-30 sub-compact cranks have the same tooth count difference between the rings. You would just need to drop the derailleur on the seat tube a little. From the pictures on the page you linked to the derailleur appears to be a band-clamp type so there should no problem moving it (some braze-on style brackets prevent the derailleur from being dropped to accommodate smaller rings).