Installed refind to a separate HFS+ partition alongside macOS 10.14, but can't set it as the startup disk
First of all, you do not need to disable System Integrity Protection (SIP) to install rEFInd. Second, you do not need use the bless command to make rEFInd the default on startup. Third, while installing to a HFS+ partition does work on a 2015 Mac, eventually this will no longer be possible on the 2018 and newer Macs. (See the recently posted question: Bless error when installing rEFInd) 

As for your specific question, recent versions of macOS will not allow rEFInd to be selected from System Preferences. However for a 2015 Mac, you should be able to select rEFInd as the default on startup by using the Mac's Startup Manager. Restart the Mac and immediately hold down the option key until the Startup Manager icons appear. Next, hold down the control key while selecting the partition rEFInd is installed on. In your case, this partition should by default be labeled the same as HFS volume. Now on subsequent starts, the Mac should boot to rEFInd.

I should also point out that if Windows is installed with the Windows Support Support software, then rEFInd may be set as the default at startup from the Windows Control Panel. This option is only possible when rEFInd is properly installed to a HFS+ partition.


  Note: The rEFInd Boot Manager can also be installed 2015 or newer Macs to either an EFI partition or a MSDOS (FAT) formatted partition. You can use the default EFI partition or on any additional EFI partition of your choose to add. If installed properly, rEFInd can be blessed by the Startup Manager.


One final comment. If you install rEFInd to a MSDOS FAT partition, then you can change the default operating system, that rEFInd will boot, directly from macOS, Windows or Linux. This can be done by using a text editor or by installing a script. (The use of these scripts is shown in this answer.) The same is true if rEFInd is install to an EFI partition, except accessing the EFI partition usually requires elevated permissions.