The Xiaomi 12T Pro or other Xiaomi smartphones in the upper price range unfortunately never have the option to expand the memory. Which is a pity, of course, because you often have to live with the restriction. There are some that also have 512 GB of storage, but they are even more expensive.
Perhaps the Xiaomi 13, which has only just come out in China, could help with the Leica components, because the sensor in the Xiaomi 13 would already be very large at 1 inch, which gives hope for good photos, but unfortunately it won't have memory expansion either.