Concrete is concrete. A large slab = expensive. A large garage = expensive. I'm seeing some water in the background of the photo which screams out WATER MITIGATION needed for whatever you are building.

You can go 'sturdy' today (which also means expensive) or you can go EXPENSIVE tomorrow (ie. jacking out bad substrate under the garage slab...'cause you wanted to save some $$$).

Never EVER go cheap on the envelope! Ever! The Envelope keeps the walls up, the water out and the ground underneath you level.

If you must take money from somewhere, take it from a gregarious roof line (I'm making an assumption here...it is for illustration purposes only - please bear with me) or from an over-the-top kitchen.

Spend money on your construction, and you will save money at the end of the day. I promise. It is easier to rip out 'builder grade' cabinets in 15 years than it is to jack out a garage slab, stabilize the home with pillars, remove the nasty fill and then pour the concrete again....

If you have a 'multi-peak roof line' then look at shaving off $10k worth of corners. If you must do a 'redesign' for the roof/attic then NOW is the time to do it. You have the forms up for the slab (which is designed to take the MASSIVE weight of a big, expensive roof (again...I'm assuming here...I don't know your build) which means it is OK with taking a LIGHTER LOAD if need be.

I know that's not what you were hoping to hear for a solution...but FOUNDATIONS are not the place to save money. Even if they are in a garage.

It does not look like an excavation, but the slab elevation was always meant to be raised from grade (floodplain elevation?). Then was the fill overlooked? As you seem aware, gravel should be used rather than any type of soil if you want best success in mitigating settling. What is the depth of fill? If deep, I would not do the pier route - at that point, might as well go precast planks and have a usable space below. I have been part of commercial projects where the parking lots were raised with stacked 'plastic' platforms (similar to septic drain tiles), but those may not be cost prudent for this project.