I'm not sure what I'm going to offer is going to help you; I'm not all that experienced in concrete work outside of what little I've done around my own house, and I've not addressed your issue specifically. But I recently had to do some concrete repair on my slab foundation, which required a lot of reading and research on the various DIY concrete products available to the general consumer. This is what I learned:
If you are looking to do a patch/repair, the product you are going to need is going to depend on the depth and extent of the repair. Quikrete has a couple different products you could possibly use:
Polymer Modified Structural Repair:
https://www.quikrete.com/productlines/polymer-modified-structural-repair.aspx
You would not have to use any bonding agents, but it is only intended to be used for patches less than 2 square feet. It can be used for partial or full depth repairs, and would be plenty strong enough for vehicle traffic once cured (which is pretty fast). I'm guessing what you need to repair is more than 2 square feet, so this probably isn't the right product.
Sand/topping mix:
https://www.quikrete.com/productlines/sandtoppingmix.asp
This stuff is intended to be used to top existing concrete so that you end up with a nice level surface. It is only for repairs less than 2" thick...any more than that, and you'll need to use a different product. Pay attention to the instructions if you use this one - if the repair depth is between 1-2", you'll want to use a bonding agent so that it will stick. If it is less than 1" thick, you'll want to use an admixture (QUIKRETE Concrete Acrylic Fortifier #8610) in place of some of the water so it will bond. I'd also recommend sealing it with QUIKRETE Acrylic Cure & Seal once the concrete starts to set up so that it will cure properly. I have personally used this product to repair a portion of my slab foundation under and in front of my front door - I chipped away about 1" of the old concrete and went over it with this to provide a nice, flat surface so I could replace some brick pavers I had to remove during the repair. It worked well, and was very easy to use, but it is worth noting that it will never see anything more than foot traffic.
Anything more than 2" of repair depth, and you'll want to use something more along the lines of standard concrete, probably something that will resist shrinking/cracking. The biggest issue you will face is getting the new concrete to bond to the old stuff...it just doesn't like to stick. If you don't want to worry about bonding, you'll have to break up a good section of what you have all the way down to the dirt and repour...how you attach it to your existing slab is going to be up to you. If it needs to be an integral part of existing slab (and I'm guessing it probably does), you'll need to use rebar. That sounds like a lot of work, so it wouldn't be my first choice.
I realize that I sound like a walking Quikrete advertisement, but it's what I had available to me when I needed it, so that's what I researched. I have no affiliation with their company. I'm sure there are other companies that make stuff that would work as well.
If you want to keep it easy, I'd guess you'll end up using something like a sand/topping mix with the proper bonding agent. It won't look pretty, but you can at least make it flat again. If it fails, you'll just have to break it up along with the old stuff, which you would have do to anyway.
