You could try this.
You will need:
a. An external drive of sufficient size to hold the contents of your internal drive
b. CarbonCopyCloner (which is FREE to download and use for 30 days, so doing this will cost you nothing).
Now, do this:
1. Connect the external drive
2. Open Disk Utility, ERASE the external drive to APFS. Close DU.
3. Open CCC, and clone the contents of the internal drive to the external. Be patient, this will take some time, when done. CCC has the ability to clone from HFS+ to APFS.
4. Power down, ALL THE WAY OFF
5. Press the power on button and IMMEDIATELY hold down the option key and keep holding it down until the startup manager appears.
6. You should see an icon for the external drive. Select it with the pointer and hit return.
7. The Mac should boot from the external drive.
8. When you get to the finder, open Disk Utility and ERASE the entire internal drive to APFS
9. Now, reopen CCC and RE-clone the contents of the external drive BACK TO the internal drive. Again, it will take some time.
10. When done, power down, disconnect the external drive.
11. Reboot and AGAIN repeat the "option-key" trick in step 5 above.
12. Do you get a good boot? If so, open the startup disk pref pane and re-designate the internal to be the boot drive.
Software update doesn't work right using Mojave on a drive formatted to HFS+.
It -IS- possible to get it updated, but doing so involves having a SECOND drive (in APFS) with a copy of Mojave on it.
You can then update the second drive, and "clone it over" to the HFS+ drive.
You'll then have and updated version of Mojave, still running under HFS+.
I don't use APFS, so that's what I do.