Shimano Tiagra - Install smaller front chainring I recently bought a used Trek Checkpoint ALR4, with which I'm very satisfied.
The gearing is ok so far for going on plane ground an light uphills, but when going on long and steeper hills it gets really hard to push which I don't want to sustain on longer rides.

It has a 50 and a 34 tooth chainring in the front and I was looking for a way to replace the 34 tooth chainring with a smaller one. But as far as I can see, I would have to change the whole groupset with both chainrings ant the cranks. 

My question is if there is a way to do this without replacing the whole crankset or at least keeping the front derailleur. I couldn't find any evidence online that this is possible without replacing everything.

EDIT:

Rear Cassette: 10x 11-34.

I've got spare a 11-42 rear cassette from my mtb, would this one possibly fit without chainging the derailleur?

Update with the solution I went for:

I changed the rear cassette from 11-34 to 11-36 (CS-HG 81-10) which worked straight out of the box without any adjustments.

For the crankset, I went for the Shimano GRX 2x10 46/30 crankset which was easy to install. I had to shorten the chain by 2 links. It took me quiet a while to adjust the front derailleur and at the point where it shifted correctly, there was some friction when shifting into the higher gears on the bigger front chainring (The chainline of the GRX is 3.4mm wider than the line of the Tiagra crankset). With no possibilities for adjustemnts left, I ended up bending the metal of the derailleur a bit. Im pretty sure every professional would have told me to buy a new derailleur, but now it shifts even better than before all those modifications and the disfigurement that I did to the derailleur is not noticeable if you don't know it.

I'm pretty happy with the gearing of my new setup.
Costs for the complete conversion approximately 120€ and 2 hours of my time.

Thanks for the help!
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).