Dare I say it......
I'm not at all convinced that road bike can be converted to an off road bike. I don't see the point of hacking two usable bikes for something which will be a neither here nor their final product. It may be best to sell the lot and re-design and re-start from scratch.
Find a nice steel Hybrid for example. Cheap as chips, and designed for such a task with appropriate geometry. With proper clearance, with proper brakes (V or canti), stronger wheels (like touring rims), invest in the lightest innertubes, and light Kevlar folding tyres. A Hybrid only falls short when it get's too technical off road or too fast on the road.
I can't even see the problem with a 26", you go slower up hill, but faster down hill. If you start with a solid Ti frame with a Weight Watchers program, Mavic Crossmax, Pace RC31 etc. you won't be far off a modern Gravel Bike in terms of weight.