Can I replace my 1 TB Fusion Drive HDD with a new SSD without splitting the Fusion Drive first ? (because splitting is not possible)

I have a late 2015 27'' iMac with 1TB Fusion Drive. Normally I was using my macOS through an SSD connected to the USB and I was sooo happy about the performance of my computer. One day it restarted itself while in sleep mode and... never be able to start again; Symptoms: Apple logo appears when I connect my iOS containing external SSD, but the progress bar never fills up (it stays blank forever) Question mark appears when I do not connect my external SSD with iOS. Command+R does not work and OPT+Command+R do not continue after some point during start up Apple diagnostics say my computer is ok I resetted my NVRAM and tried all possible startup key combinations including Command+S (I cannot reach to Terminal (some weird data log appears and no place to write a command) Apple authorized service cannot do anything since my computer is considered as vintage I talked to the apple expert, and he said most probably my HDD is dead (it is silent now and I do not hear it anymore). Do above symptoms root from the dead HDD?, even all my iOS data is in my external SSD? Will replacing my internal HDD to a new internal SSD solve the problem? I cannot split my Fusion Drive since I cannot reach to the Terminal app, will that be a problem? What symptoms do you think I will have right after replacing my HDD with a new Sata3 SSD? I like my computer and want it back, thanks to all in advance