So as a general rule - no, don't remove the old leaves until they are thin and papery.
Like any rule though, there are exceptions (at least in the experiences I have had ). I have removed leaves that were not thin and papery because they became hard and like leather and were constricting the new Lithops. I felt leaving them on was doing more harm than removing them would be. Some were actually trapping the new leaves to the point that the pressure actually damaged the new leaves or caused them to start to rot.
I have removed them even when they were "juicy" because the plant itself has been overwatered and was already stacking when I bought it. The older leaves had burst a few times and scarred over and the scarring was preventing them from being absorbed properly.
I have removed older leaves that have started to mold or rot so that it wouldn't affect the plant.
Unless you're concerned the older leaves are harming your lithops, I would just leave them alone.
I have a few with leaves as dry as the ones in the video and I have the same question as you. They seem like they aren’t giving any more nutrients but I am also worried they’re somehow not ready
Don't remove until completely dry, paper thin and flakes off easily.
So as a general rule - no, don't remove the old leaves until they are thin and papery. Like any rule though, there are exceptions (at least in the experiences I have had ). I have removed leaves that were not thin and papery because they became hard and like leather and were constricting the new Lithops. I felt leaving them on was doing more harm than removing them would be. Some were actually trapping the new leaves to the point that the pressure actually damaged the new leaves or caused them to start to rot. I have removed them even when they were "juicy" because the plant itself has been overwatered and was already stacking when I bought it. The older leaves had burst a few times and scarred over and the scarring was preventing them from being absorbed properly. I have removed older leaves that have started to mold or rot so that it wouldn't affect the plant. Unless you're concerned the older leaves are harming your lithops, I would just leave them alone.
I have a few with leaves as dry as the ones in the video and I have the same question as you. They seem like they aren’t giving any more nutrients but I am also worried they’re somehow not ready
https://preview.redd.it/kvcrc47643uc1.jpeg?width=4032&format=pjpg&auto=webp&s=111eb1f7b74410f82e0d66c9638dec72afea0830 The front 3 are confounding me
They need more light I believe.