Your HFS+ partition and the TM APFS partition+volume are separate partitions. So if you reformat the HFS+ partition as APFS it will change the partition to an APFS container with a single volume inside it. Thus you get two partitions, each formatted as an APFS container. There is no automatic putting both volumes inside one container.
Depending on the layout of your disk, you may be able to delete the HFS+ partition (after copying the contents somewhere else) and expand the existing APFS partition/container to fill the whole disk. If so, you can then add a second volume to the APFS container. You will then have a single APFS partition/container with two volumes - the TM volume (unchanged) and an empty new volume.
But you may need to erase the whole disk and start again with a new APFS container with two new volumes (one for TM and one for data). That will mean starting again with your TM backup.
Adding the output for
diskutil list to your question would help.
I following your links back to this question ghost files—now you see 'em, now you don't? From this I can see that your corrupt HFS+ partition is first on the drive. In which case, deleting the HFS+ partition will leave an empty space and, though I have not done it recently, Disk Utility has never allowed me to move an APFS partition to the start of the disk which would have allowed you to expand it.
So I believe your choices are to:
- Erase everything and start again (which I know you don't want to do) creating a single APFS partition, or
- Create a 3TB APFS partition to replace the deleted HFS+ partition.
The latter will leave you with 2 APFS partitions/containers (3TB and 5TB) with the 5TB container having the one TM volume. You can add volumes as you wish to each container.