upgrade fusion drive from 128gb to 1tb and remove hdd

A Fusion Drive is confusing!

Its made of two physical drives a SATA HDD (spinning rust) and a PCIe/NVMe blade SSD.

So what you did in breaking the fusion drive set and then replacing the HDD with a SSD got you some performance but you still have the PCIe/NVMe interface which presently has a wimpy drive size-wise as its function was a cache drive to speed up your HDD’s access not large enough to do much anything else.

So you could replace the blade drive with something larger iMac Intel 21.5" EMC 2638 Vervanging van de blade SSD going with 48GB or larger! Then make it your boot drive and holding your apps. Then leaving the rest of the space unused for systems virtual RAM, caching and depending your apps scratch space. Here in this config we are using the SATA drive SSD as your data drive to hold your work and other media content.

TimeMachine requires an external drive to be your safety line when your system goes down. Having it internal would prevent you from getting your system serviced and still be able to access your stuff.

As an example I have a few external drives strictly for backups as a photographer I hold my projects on external drives and only hold my current work on the system with my iTunes music.

As we don’t know your workflow is we can’t give you much more.