Install Ubuntu as Dual Boot encrypted alongside Windows 10 encrypted

I have a machine with a 1TB SSD. 800GB of which are consumed by a bitlocked Windows 10, 200GB are unallocated and thus free space.

My goal is to set up Ubuntu on that 200GB space, fully encrypted.

To achieve this, I first created a 500MB /boot partition (EXT4) in the free space. Afterwards, I created a “physical volume for encryption” using up all the rest of the free space. Inside of this crytpo-wrapper, I created a partition for /root (75GB) and /home (~125GB), both using EXT4.
When clicking install now, it shows a message saying that /boot and /home are too small. When clicking continue anyway, the installation crashes.

What can I do? How does this need to be set up?