This is madness, but since this is a hobby project and not a production server, there is a way:

• Shrink the filesystems on the existing disks to free up as much space as possible, and shrink their partitions.
• Add a new partition to each of the three disks, and make a RAID5 volume from those partitions.
• Move as many files as possible to the new RAID5 volume to free up space in the old filesystems.
• Shrink the old filesystems/partitions again.
• Expand each RAID component partition one at a time by removing it from the array, resizing it into the empty space, and re-adding it to the array, giving plenty of time for the array to rebuild.
• Move files, shrink the old partitions, and expand the new array partitions as many times as needed until all the files are moved.

This could take several days to accomplish, because of the RAID5 rebuild times. The less free space, the more iterations and the longer it will take.

If you can assign a second IP address to the network interface, then just do so, and bind the docker container to one, and Adguard Home to the other. Otherwise, the reverse proxy based on the server name is the way.