The material costs for a project like the F-35 are always going to be a small portion of the overall cost. Most of the costs are in manpower. I read somewhere that for every dollar American spends on a top secret project, they spend $10 on security for it. That may be exaggerated but I think itThe thing is, Japan will never be a proper replacement for Russian titanium metal to the industry.
Like I said, a major part of the cost to get titanium metal is energy, and Japan has next to no native sources of cheap energy they can use to produce titanium metal. Typically the industry uses the Kroll process and this requires many melting steps at extremely high temperatures and a step where it goes through an electric arc furnace as well. As global energy prices go up, you will see the price of Japanese processed titanium increase dramatically, let alone all those customers competing for their supply. The idea these clients will easily switch production elsewhere is, I think, kind of naive. It takes many years to get one of these plants operational and years more to optimize the processes to get it to be efficient. The processes need to be tuned to the ore you are processing among other things. This is typically a painstakingly slow process with much trial and error. You might be looking at 5 years at least to get the production at a decent clip. I heard someone once say 8 years. So this would be like the Russians just saying they can manufacture their own semiconductors. It just isn't as simple as that. I think they will just continue buying either Russian or Kazakh titanium and pass it as something else.
I still remember a couple of decades ago there was much hype about FFC Cambridge process replacing the Kroll process and making titanium as cheap as aluminium. It did not happen. They never got it to work reliably enough in practice.
That said I think you're right. Japan will just buy Russian titanium via the Kazakhs or Mongolians and then sell it on to Boeing.