And with the DIY option, you can skip making a Fusion volume. IMO separating files and system/apps/temp between HD and SSD is working great, and allows you to make a clone of your system on the HD in case one drive breaks.
Fusion will make roughly the same split of files, but it is "cleaning the house" on weird times in the background.
In the EU, self-replacement does not void warranty. Don't now where you reside.