You can also chamfer the edge of the hardwood. Probably easier than chamfering the marble unless it is a small chamfer. A larger chamfer over a portion of the threshold is common for differences as small as 1/4 inch. The threshold is simply turned into a small ramp to ease the transition and eliminate any edges.

Metal reducers are typically used for carpet transitions, but they can also work well for wood/solid transitions. I used them for hardwood to sheet vinyl. The drop was a about 1/2" and they work great. Most of the metal ones are very thin, so your feet can easily slide over them. Some use a heavier gauge of metal than others, and some have a rounded over edge. I found the commercial ones in long lengths to be the best combination of heft and price. The metal strips are made to be nailed. I had to use some 1/2" thick wood shims underneath the metal transitions in my room to room gaps. They have held very tight with over a year of family use and nobody trips on them.

Thanks, Sophie! We are already too far into this process to go back to the joists, so we are just trying to find a thinner stone that will work for what we want. We did manage to find a 1/2" option, which I think we will go with and just do a threshold reducer in the two doorways to compensate for about a 1" height difference between the rooms. The rot was actually due to a leaking refrigerator left on in the house by the previous owner for over a year when no one was living there. We purchased the house as-is, so we actually knew that this was going to be a problem when we got it. We are doing all the renovations DIY because we can't afford to hire anyone to do them and my father-in-law built a section of his house himself and is helping us. That is good information to know for the future though, thank you!

Are you going to be installing new flooring on both the old wood and the new concrete? I will assume there is some reason that the new slab/concrete wasn't poured higher (Like a doorway that we can't see or something) than it was to match the existing sub floor height. Also based on your picture... it looks like that portion of the slab concrete floor needs to be flattened before you could install most any type of flooring. I see the concrete elevation drop off quickly at the right side of the picture. I would carefully check you newly poured slab out to see if you have other areas with dips/humps and fix those areas before you think about installing new flooring. Chances are you need to do a little (or a lot) of self-leveling/flattening of the slab before you install the new flooring which may gain you a little elevation on the slab side. You will need some sort of reducer to transition from the wood side to the slab side. Typically the laminate reducers wont necessarily work for that height difference. So I look at the available pre finished hardwood reducers which will be closer to what you need for height difference. Usually they have overlap reducers that have 3/4" clearance which will get you closer. You can always make your own reducer from a piece of hardwood with a table saw, sander and some time. Also please check with the manufacturer of your particular flooring product about their specs for the flatness of the floor, moisture content of concrete/wood subfloor, acclimation of material before you go for it. Wood flooring products like laminate are especially prone to post installation issues if you ignore things like leveling the floor, moisture content of concrete etc.