i can see your mac computer's specifications page here :
iMac (Retina 5K, 27-inch, 2019) - Technical Specifications
( i think, you customized during initial purchase )
this mac computer can use max total 64 GB RAM , so upgrade RAM into that.
you will need 4 pcs of 16GB DDR4 2666MHz SODIMM RAM Modules.
16GB modules are not over-costly now.
if you replace HD-Fusion drive, with same another new one,
main problem still exists & will exist : HD-Fusion drive is a HYBRID category drive, with small amount of faster SSD, & majority/larger size of drive is slow HDD based.
HDD (mechanical & magnetic) are slower than SSD (electronic semiconductor circuitry based).
SSD can be 3 to 10 times more faster than HDD.
HDD type of drive's memory bit can crash more often under environmental stress conditions, such as over-heat, sub-zero temp, high-altitude, etc etc etc, and very less tolerant to SHOCK or DROP from higher elevations.
HD-Fusion or HYBRID category/type of drive, will again soon FAIL.
it happens more often when you have HDD.
SSD has extra storage pool memory, when a used storage memory bit becomes faulty, then SSD logic circuit allots another fresh bit from that pool.
please get a full SSD drive, 2TB SSD drive, SATA-III based.
that will make your computer working very fast, and run in stable manner for very long time.
if SSD drive quality is better, then it will perform better for long time.
as the computer has two Thunderbolt3 ports, if your monitor does not have a TB3 or TB4 port, then buy a converter/cable/adapter that can convert monitor's DP (or HDMI) port into a TB3 port based cable, then video will work even better.