OP asks:
"Is there a way I can downgrade it to El Capitan?"
Yes.
It will "take some work", but you can "get back to where you once belonged" if you really want to go there.
Let's get started.
You're going to need a few things:
- A copy of the El Cap installer. You can get it here:
https://support.apple.com/en-us/HT206886
- A USB flash drive (try 8gb, if that doesn't work, try 16gb)
- The small free app named "Boot Buddy". Get it here:
https://sqwarq.com/boot-buddy/
You're still running High Sierra, is this right?
If you have stuff on the internal drive (still running HS) that you want to keep, you need to do this as well:
- Have at hand an EXTERNAL drive that's capable of holding everything on your internal drive
- Download CarbonCopyCloner from here:
http://www.bombich.com/download.html
(CCC is FREE to download and use for 30 days)
- Use CCC to clone the contents of your current (HS) install to an external drive. MAKE SURE you choose CCC's option to clone HS to an HFS+ volume (NOT an APFS volume!!!!!)
NOTE: If you DO NOT wish to keep anything on the internal drive, you can skip the CCC clone steps above. But realize that everything is going to be WIPED OUT by the procedure below!
Now, do this:
- Erase the USB flash drive to "Mac OS extended with journaling enabled"
- Use Boot Buddy (and the El Cap installer) to create a BOOTABLE USB flash drive with El Cap
- Once done, REBOOT to the flash drive installer (reboot and immediately hold down the option key until the startup manager appears. Then select the USB installer with the pointer and hit return)
- The flash drive will open to the OS installer but DO NOT INSTALL YET. Instead, quit the installer
- Open Disk Utility and ERASE the internal drive to "Mac OS extended with journaling enabled". NUKE IT BACK TO ZERO.
- Now quit Disk Utility and re-open the OS installer.
- Let the installer do its thing -- this may involve reboots and take awhile.
- Once done you should boot up to the setup page for the new install
At this point, begin setup.
Setup assistant will ask if you wish to migrate from another Mac or drive.
I don't think setup assistant can "bring over" stuff from an install that is "newer than" El Cap. You could -try-. But don't count on it.
IF there is stuff on the backup drive that you still want to bring over, you can still DO IT MANUALLY.
This also involves a little work, but I'm not going into that yet.
Your first priority is to get El Cap installed.
Do that, and get back to us.
We'll be waiting to hear from you.
Print out these instructions and "check them off" as you go along!