Late 2015 iMac 17,1 128SSD + 2TB HDD confusion.

I agree with @Strontium90's guidance regarding fixing the split Fusion drive. This will certainly increase the performance of the computer. As they posted, the process will also erase both drives, so be sure you have a backup of any important data that you currently have.

Here is Apple's guidance for fixing that split drive: How to fix a split Fusion Drive - Apple Support

Before you repair the Fusion split, I'd suggest that you create a bootable usb installer for your current version of macOS. It'll make reinstalling the OS much easier when you're ready to do that.

Create a bootable installer for macOS - Apple Support

Maintaining Home folders on anything but the startup drive is complex and risky. It's best to leave these folders exactly in their default location as the macOS installs them.

Is there a reason why you split the Fusion Drive into individual drives? If you rebuild the Fusion Drive (all data will be lost), then you will have a single boot drive, not a boot and a secondary.

And no, there is no workaround for the Apple services and home folder location that I am aware. This will also impact photo sync as a Photos library on a secondary/external drive cannot participate in cloud sync.

Consider rebuilding your Fusion Drive. This will resolve your problem. However, beware. A 2015 model is already a decade old. If that rotational drive is original, you are way past your mean time before failure. Make sure you are using a reliable backup.

Hope this is helpful.

Since your iMac is 10 years old what I'd do is get a Thunderbolt 2 with a 1 or 2 TB SSD, install the system on it, boot and run from it. Then you can add additional user accounts with no issue of lack of storage. Then when you upgrade your iMac you can take the external SSD with you. If budgets is of a concern you can save some money with this setup at 500 Mbs:

Just some food for thought.