You can have as many bootable disks as you want (and have storage for), and can select and boot from any connected, though only one startup device can be the default.
Usual reason for these trouble reports is a split Fusion, or write-heavy activities, or a failing HDD. A failing SSD is also possible, though less common.
Fusion operates at HDD speeds once the SSD cache is overwhelmed.
The size of the SSD varies, and some of the Fusion SSD configurations were tiny.
Run Diagnostics or DriveDx and check the HDD status: Use Apple Diagnostics to test your Mac - Apple Support
To reconstitute: How to fix a split Fusion Drive - Apple Support
To answer your questions, yes, you can have multiple bootable devices and any that are bootable can be set up to boot.
To create a bootable macOS device, you do need to for at it as GUID GPT partitioning and (for any recent macOS) APFS file system, and then load macOS.
How to Setup and Use an External SSD as your startup disk… - Apple Community
An external SSD on a fast bus (USB 3.x or Thunderbolt) is usually preferable to a Fusion, particularly for write-heavy workloads.
Change your Mac startup disk - Apple Support
Another detail to check is memory, particularly if you have the 8 GB configuration, or are making heavy use of what you do have. Available memory in an iMac 27” can usually be upgraded. The SSD will usually be a more significant gain, then memory if you’re in an 8 GB configuration, or have more memory installed but are using it all.
Install memory in an iMac - Apple Support
And technically, if the SSD and HDD within that Fusion is all working, and if the SSD is big enough, you could split and install and boot from either or both. The SSD probably isn’t big enough for anything useful; not past a bootable installer, or such.
