Create Fusion Drive on SSD with Bootcamp

I have not tested running CS Fusion on a Mojave or Catalina. It should work, but macOS may force you to convert the CS to APFS Fusion which can cause issues.

To install Windows on the SSD part,

• You will need to remove the SATA connector to the HDD. Do not remove the SATA connection from the Logic Board side, but the disk side.
• At this point macOS can only see the SSD which will be partitioned by BC Assistant if the Install option is enabled. If you need the additional iTunes Media partition (not an APFS volume) then you will need to manually partition the disk, and not use the BC Assistant. You can still use BCA to build the USB2 Flash drive installer using the Create and Download options enabled.
• Whether you partition the disk via BCA or manually, you will need to use Alt/Option to boot from the Windows icon on the USB (do not use EFI Boot).
• Once booted from the USB installer, you can select the designated Windows partition and install Windows and test and verify. You may have to manually create the Hybrid MBR for BIOS mode using GPT Fdisk, if you see a GPT vs MBR error, or your disk is manually partitioned.
• After Windows is fully installed, you will need to reconnect the HDD and then create the Fusion LVG and LV using Internet Recovery and diskutil cs commands.
• At this point, if you restore Catalina from a TM backup, your CS Fusion will be forced to become APFS Fusion. This should be fine, since you already have the three parts on the SSD and CS Fusion spans the slice you used to create CS Fusion on the SSD and the full HDD. Be aware that CS Fusion has a separate Recovery part on each disk slice, but APFS combines these into a single Recovery APFS volume.
• The sizes for SSD you have chosen should work. Your SSD should have
  • disk0s1 - EFI
  • disk0s2 - macOS SSD part
  • disk0s3 - Windows part (no CS Recovery HD also called Apple_Boot - it should be manually removed)
  • disk0s4 - iTunes Media
• The HDD is disk1 (by default), and it should have
  • disk1s1 - EFI
  • disk1s2 - designated CS Fusion slice
• The CS Fusion drive should have
  • diskutil cs create macOS-LVG disk0s2 disk1s2
  • diskutil cs createVolume macOS-LVG-UUID jhfs+ "Macintosh HD" 100%
• One option you may want to consider, to avoid issues, is to make the iTunes Media an APFS volume, instead of a partition, which is a bit cleaner. You will not need to remove Recovery HD and iTunes Media backup can be restored after the Catalina TM restore.
• Windows Restore can be problematic, because Windows Restore assumes it has full control of the disk, which is not tru in your specific case.

Assuming you have a 2012 MBP, my recommendation is installing the SSD in the main disk bay (not the Optical drive bay). Do not put Windows on the disk in Optibay, but you can use it for file storage. Building a Fusion drive between the SSD and a partition on the Optibay HDD can cause performance issues. Normally the HDD partition for Fusion drive is larger than the corresponding macOS partition. You also need to account for CoreStorage Fusion vs APFS Fusion, which depends on the macOS version. Mojave and later support APFS Fusion, while older macOS versions support CS Fusion.

Glandrim wrote:

But the question still is if the creation of the fusiondrive will crash the bootcamp or if i can use it afterwards.

No, as long as you use the correct disk slices to create the Fusion drive (CS or APFS), you should be fine. APFS uses the -main and -secondary qualifiers when creating the APFS Container. APFS fusion does not allow adding physical slices after the Container has been created, but CS Fusion does (see https://blog.fosketts.net/2011/08/05/undocumented-corestorage-commands/ for reference). For example,

diskutil cs adddisk
Usage:  diskutil coreStorage addDisk lvgUUID NewMemberDeviceName
Add a new physical volume to a CoreStorage logical volume group.
Ownership of the affected disks is required.
Example: diskutil coreStorage addDisk
         11111111-2222-3333-4444-555555555555 disk4s2

and

diskutil apfs adddisk
diskutil: did not recognize APFS verb "adddisk"; type "diskutil apfs" for a list

CS Fusion allows more than 2 disks, while APFS Fusion does not.

But i do not understand why the size of the drives (SSD 1TB and HDD 0.5TB) matters for performance.

A Fusion drive allows blocks to move from the faster to the slower disk. Unused storage blocks are moved to the slower disk, which requires the slower disk to have more disk space, than the SSD part. In macOS Catalina, APFS will not let you create a Fusion Container if the 'slower' part (HDD slice) is smaller than the 'faster' part (SSD slice) in terms of disk space.

For example,

diskutil apfs createContainer
Usage:  diskutil apfs createContainer <disk> [<disk>]
        diskutil apfs createContainer -main <disk> [-secondary <disk>]
        where <disk> = MountPoint|DiskIdentifier|DeviceNode
Create an empty APFS Container. You can then add APFS Volumes with the
diskutil apfs addVolume verb. If you specify two disks, then a "Fusion"
Container is created, with the performance usages assigned automatically
unless you use the -main and -secondary options, in which case, the secondary
disk is assumed to be on "slower" hardware. The secondary disk is often used
to store associated "auxiliary" data, such as a Boot Camp Assistant partition.
Ownership of any affected disks is required.
Example:  diskutil apfs createContainer disk0s2

I personally prefer the CS implementation, not the APFS one. Also, APFS is a very buggy file system/container system. CS or ZFS is a much better alternative.