Reporting back a day later. I'll be complete so this information can possibly help others.
iMac (Retina 5K, 27-inch, 2019)
Processor 3,6 GHz 8-Core Intel Core i9
Memory 48 GB 2667 MHz DDR4 [by manual upgrade]
Videocard Radeon Pro 580X 8 GB
Harddisk Fusion drive 3TB
External hard disk SanDisk Extreme SSD 1TB
The iMac came with macOS (Catalina?) pre-installed on the Fusion drive.
I upgraded to Big Sur on the Fusion drive but the system became stuttery. It was suffering short halts and made working super annoying. I read a lot of info online and found a promising cost-efficient solution in booting from an external SSD.
Until last week:
I had Big Sur installed on and booting from the external SSD drive, attached through Thunderbolt 3. This was working very smooth, the external SSD was notably faster than the internal Fusion drive. The only downside was that during booting, the SSD was still powering on and therefor could not be found at boot. To work around this I had to hold Alt/Option every boot to load the boot manager and pick the boot drive manually, because usually it would be live just half a second too late for the normal boot sequence. Once booting however, it was fast.
I upgraded to Monterey on the external SSD, installation went ok, but the reboot after finalizing took 3 hours (!!!) to complete. I only gave it this much time cause I didn't want to crash it during installation, at some point I didn't expect it to continue, but in the end it did. Monterey was installed and booting, however, every boot since took about 30 minutes. Super slow.
I read online that there are issues with Monterey on non-factory hardware. So I decided to try installing on my internal Fusion drive rather than on the external SSD. I had however used the internal Fusion drive as a data disk, and had to backup 2,5 TB of data before I could do this. I found 4 TB worth of disk space spread out over old USB drives, and backed up everything manually on these drives.
After completing and securing my backup I created a Monterey portable installer following one of many (easy) guides on the web on one extra external USB drive. This was done in a few minutes.
I shut down the machine.
I unplugged all external drives and third party apparatus.
I connected the Monterey installer USB drive and booted the iMac holding down Alt/Option to then pick the Installer as boot drive.
The installer boots to a welcome and I used the here provided Disk Utility to wipe all data from the internal Fusion drive.
Now ready to install Monterey on the internal Fusion drive.
The installation succeeded in less than 20 minutes. Nice, this is faster than it went before when upgrading Big Sur to Monterey on the external SSD.
Booting Monterey on the internal Fusion drive goes as snappy as one would expect, booting in a few minutes or less!
Now, I read somewhere that people having issues with slow booting Monterey on SSD drives (mostly MacBook users) fixed their issues by installing on the original hardware and THEN reverted back to their upgraded non-factory hardware. The idea is that the Monterey installer recognizes factory hardware and only then upgrades EFI firmware, once this upgrade is done, Monterey is supposed to run snappy on whatever hardware installed.
So for the ultimate test, I reset my iMac and boot again from the external SSD drive with a clean install of Monterey on it, to see if this installation (that was still on the drive) would now load faster.
Well, it didn't. It was still taking about 30 minutes to boot from the external SSD.
So, I'm keeping my Monterey installation on the internal Fusion drive. I've now migrated my installation from the external SSD to the Fusion drive using the Migrate assistant in Applications/Utilities. This worked as a charm.
[Why? Cause I had in the meantime re-installed and configured all my apps etc]
I wonder if I'll keep my boot speed on the Fusion drive installation as I move forward copying back terabytes of backup data from the external USB drives to the internal Fusion drive, but I'll have to see.
For now: I strongly suggest installing Monterey on your stock hardware! The Fusion drive (I have the 3 TB variant) seems to work great so far!