There are some issues here:
-- apfs file system
-- encrypted startup drive.
I suggest you get an external drive and backup your drive. I suggest using ccc. You may need to unencrypt your startup drive. Two external backups are best.
The easiest way would be to get an external drive and copy all the data to the external drive. Reformat you internal drive. copy data back to original drive. For the startup partition, you will need to use a backup utility.
"Clone, synchronize, backup. Schedule and forget it."
"SuperDuper is the wildly acclaimed program that makes recovery painless, because it makes creating a fully bootable backup painless."
You can use Disk Utility to do the copy too. But, that's some assembly required, since I do not know how to do this.
Another way would be to buy a partition manager. Delete the backup partition. resize the data partition. Move the data partition or insert a partition between your Startup drive and your data partition. delete the splitter partition. Use disk utility to expand you startup drive. [ I didn't see that a third party partition manager supported apfs. ]
I suggest you have a full backup of your data.