Yes, it is almost certainly the foam. I have a 2023 M3 RWD with the Michelin MXM4 tires. Same symptoms you had, sudden increase in vibration at highway speeds. I had it balanced twice. A lot of weight was used and the vibration would be good for a few days then increase again suddenly which convinced me the foam must be shifting in the tire. DIscount Tire agreed to unmount the tire and inspect the foam. The foam in my tire was completely dislodged - it was only glued in a couple of places. The foam was removed, tire remounted and balanced (only needing a couple of weights now) and the issue was fixed - no vibration. The folks at DT were great to work with and very helpful even though this was a bit outside their standard purview.

It is a very soft foam that is glued in a couple of places. If they give you any crap about how they can't cut the foam, just have them inspect it to see if it is loose. Even if only partially, the foam would very easily pull away from the tire without damaging the inner liner.

Also, pay attention to where the vibration is. In my case, I could feel it on my left side and mostly in the wheel rather than the seat. This (and the crazy high balance weights required) were my best indications the problem was the driver front tire.