Is there anything i should know before i update my retina iMac Fusion Drive (Sierra) to Mohave?

Time Machine will continue to work as it always has, and will provide recourse in the event you need to "undo" the macOS upgrade.

Sierra did not convert iMacs with Fusion Drive to APFS. If you were to restore the entire Sierra system from its Time Machine backup, the volume will revert to HFS+.

A backup is essential, but one and only one backup is not sufficient. Ideally, create another, redundant Time Machine backup prior to upgrading macOS.

Sure. Use Disk Utility's Restore function. It creates an exact copy.

For instructions: https://support.apple.com/guide/disk-utility/restore-a-disk-dskutl14062/mac

...I can’t afford a new hard drive at the moment)

I understand, but an external hard disk drive doesn't cost much more than a tank of gas these days. Only you can determine the value of your data though.

Follow these instructions to erase it: How to erase a disk for Mac.

To install macOS on that device, follow these instructions: How to reinstall macOS from macOS Recovery.

I am going to make a duplicate of my hard drive. however, i want to do a clean install so this will be mostly a backup in case that doesn't work.

I think I understand what you want to do, but to be clear those are two different goals. A duplicate of a previously configured system and a "clean install" are mutually exclusive. Apple does not define or use that terminology which leads to confusion.

In addition to Time Machine the only thing I do with any regularity is to use Disk Utility's Restore function. There is no need to completely erase a system just to upgrade macOS. In fact there are valid reasons not to... but it's always an option if you so choose.

A "clean install" if you want to call it that is only justified if I'm selling or giving away a Mac. That's a different subject altogether, but if that's what you want to accomplish then follow those instructions under Why to erase a disk in the link above: How to erase a disk for Mac.

Perhaps this will help: Upgrading macOS without fear. It describes what I do, but it essentially comes down to "create a Time Machine backup or two before upgrading." It's also what Apple recommends... Step 2 of their upgrade instructions.

I thought doing a clean install once in (long) while is a good thing?

How do these rumors get started? Perhaps it's a Windows thing, like so many other indignities its users endure.

I have performed nothing but upgrade installations on all my Macs, forever, going back to the original OS X public beta, which I also installed as an upgrade from OS 9, which was upgraded from OS 8... etc.

For new Macs I use Migration Assistant, which migrates all possible compatible code from the Macs they replace. Needless to say I have no idea what programs you require, but to install everything anew that I require would be untenable.

To cite a trivial example if you perform an "erase and install" you might find it impossible to use old printers, rendering them landfill fodder. My original Apple ImageWriter still works. It's over thirty years old. Upgrading macOS is what continues to make it viable, whereas an "erase and install" would render it literally impossible to install with Mojave. (Side note: it's not long for this world though, due to the impending sunset of all 32 bit code).

Over the past decade or two I dragged incompatible programs originally written for the Motorola 68000 or PowerPC code to the Trash. That describes the full extent of any "housekeeping" I've ever done.

(although that probably has more to do with my lack of ram---i have 8gb.)

What lack of RAM? macOS runs perfectly fine with half as much, and requires only 2 GB. Another urban legend debunked. The difference is that recent macOS versions have grown to rely more upon virtual memory than before, so if you have a traditional rotating hard disk drive it's going be slow, regardless of how much physical RAM is installed.

If your Mac relies upon a rotating hard disk (as opposed to flash memory) you can install 64 GB and it will still be slow.

Bear in mind these facts: Apple has been designing iOS for flash memory since its inception, that macOS has been inexorably becoming indistinguishable from iOS (they share a lot of completely identical code and use completely identical file systems), iOS devices traditionally had limited memory resources but have always used flash memory for storage, and that nearly every new Mac uses flash memory. For evaluation purposes, go ahead and install Mojave on an external hard disk drive (and I encourage you to do so) but expect it to work slowly.

To address the ability to “undo” the macOS upgrade should you want to do that, yes both options convey that ability. Either one can also be chosen as the User Account data source for Migration Assistant, should you choose to completely erase the Mac, install macOS, and reconfigure the system from the ground up if that’s what you want to do.

Strictly speaking though they’re not the same thing, and each method has its advantages and disadvantages. If you’re particularly cautious, use both.

Just be sure your Time Machine backup is current. Otherwise, nothing to be concerned with. I upgraded a Late 2012 and Late 2015 5K iMac with Fusion drives without a problem.