If you want a real speed boost, split your Fusion Drive and replace the crappy Apple SSD part that comes with it. You must make a backup first (even two), because you will lose all the data on the internal device. I did the same on my daughter's 2017 iMac, and now it's truly a gem, very snappy (with Ventura... I don't upgrade macOS before xx.3 since years). To update iMac's FusionDrive I used a 1TB WD SN 570, but now you can find the SN 580 at more or less the same price (around 60 euros for 1TB). You will need an adapter to use standard M2 NVMe drives: I used a cheap sintech, purchased on Aliexpress for around 5 euros, together with the Display strips kit, necessary to re-mount the iMac (less then 2 euros... I suggest you to take two or even three kits).
In conclusion, now my daughter's iMac has a 1TB NVMe SSD and a 1TB hard drive for TimeMachine backup.
It's a bit of a chore, but with the guidance of iFixIt it's doable, and it's really worth it.