Looking for ideas on how to repair my garage floor/transition between the driveway and the garage.

Some notes on the existing garage:

1. It has a continuous 48″ deep footing and buried foundation wall all the way around.
2. There is a drain at the center of the garage, and the entire garage is pitched to that drain.
3. When they originally poured the slab, they embedded a piece of steel (possible just angle iron?) where the concrete transitions from “foundation wall” to “floor slab” at the entrance. This served as an area for the garage doors to terminate into. All of the steel is rotted out and the concrete is cracked, so I’d like to get this all tuned up and flat for the new door to seal against.
4. We’ve done structural work in the garage to go from 2 doors to one big door.  (A good seal is important because we’re insulating and conditioning the garage.)

I’m thinking I’m going to cut back 18″ or so into the garage, and pin into the existing slab with rebar. My question really pertains to how I will address the joint between the new slab that I pour, and the existing foundation wall.

Do I try to recreate the little recess for the garage door? If so, how do I do that? More angle stock? How would I form it?

Or do I just pour up flush to the top of the existing foundation wall?

Will the garage door seal poorly if the concrete that it terminates to is not flat and level?

If I make a continuous pour from 18″ inside the garage to outside the garage, will I run into issues because half of that new slab piece is conditioned in winter and the other half is out in the cold? (located in CT)