This is a tricky one.
So, to summarise, you wish to use all of the available space on the external drive to be used by the
disk6 container. One would imagine this can be done by deleting
disk5 and expanding the
disk6 container. However this isn't possible, and it's due to the underlying physical partitioning structure and its sequence. A container disk is analogous to a physical partition.
Note also that although you state
TIME are the same size, they are sharing the free space in
TIME only reports a size of 933k, it leads me to think you may not have used it yet, which bodes well for the following procedure.
disk5 is sequenced before
disk6 in the GPT, even if you were to delete
disk5, you cannot extend the start point of a partition backwards to claim free space. If it were the other way round you could extend the end of the partition you keep into the free space to the end of the disk.
A solution would depend on how much data you have already backed up in your Time Machine volume (see above), because I suspect that will need to be sacrificed to make this work. eg:
- Create an APFS volume
- Copy the data from
EXTERNAL to this new volume
EXT-COPY. This is possible because there is enough free space in
disk5 (300GB free > 215GB used)
- Delete the container
- Extend the
disk5 partition to use all the space
- Create a new APFS volume
disk5, and assign as your Time Machine device
Bear in mind that you could copy any existing data from your TM backup to yet another external disk of comparable size, and then restore later. But it's not guaranteed that it would survive the process and that TM would use it. This is because TM backups are treated quite especially and are generally read-only. I have no experience on doing this but you might need to do a disk image copy. Perhaps someone else could provide guidance on that.
Note, I have not tested this, but it might be worthwhile to setup a small scale equivalent partitioning structure on another external disk to thoroughly test my suggestion. It need not be very big, and doesn't need to contain much, if any, data. All you are testing is the workflow of managing the partitions.
diskutil list. Currently none of the partitioning and filesystem details are clear enough.