Unclear which of all the different LaCie Rugged HDD or SSD models you are talking about. A thunderbolt one is not going to work (as Thunderbolt) if you cannot chain the Thunderbolt devices you have. But since the Lacie Thunderbolt Rugged drives are only SATA internally if you have a Thunderbolt+USB model you can run them as USB 3 and not lose much.
If you are out of physical USB ports on the iMac then I would stat by getting the OWC Thunderbolt 3 Dock I mentioned.
https://eshop.macsales.com/item/OWC/TB3DK14PSL
Make one change at a time... (while I said what I'd do :-O if thing are woking OK now you might be better off just adding SSD samples to that USB SSD first and seeing how things work). You have a lot to work out to decide if you want a "best possible" setup... from just splitting the Fusion drive to upgrading to a larger internal SSD... wether any of that is worth doing depends on where you are trying to get to.