What are the drawbacks/benefits of having a split fusion drive?
I think the first thing is to define what the purpose of a fusion drive is. A fusion drive was created to so you could get the performance benefits of an SSD while simultaneously getting the value of and capacity of a spinning magnetic drive.
“Fused together” you get a single virtual drive out of two very different physical drives - a small, but fast solid state drive and a slow, but large magnetic drive. Compared to a single solid state drive, it’s nowhere as fast but then again it’s nowhere as slow as a magnetic drive, but it is convenient because it manages where data resides - commonly used files are stored on the SSD and less commonly used files on the magnetic one.
Splitting it, you end up manually managing what goes where, but now the SSD is unburdened by the slowness of the spinning media. You will get a speed increase of the files you put on it. As far as files you “archive away,” they get to reside on the slower magnetic one, but since they’re rarely accessed, speed is not of a concern.
This has been tried countless times before Fusion dives existed. Everyone that I have known that tried to install an OS on a smaller, faster drive and the data, home directories, and applications stored on a larger, slower drive have abandoned this because it ended up being a royal PITA.
Go "All In"
IMO, “messing around with it” is a waste of time because you end up with less than what you started with. If you’re going undertake reconfiguring your fusion drive, go “all in”. Open the iMac and swap out the spinning media for an SSD. If you really want to step things up, change out that smaller PCIe SSD for a larger one as well. Depending on your budget, you could have an 8TB (or more) Fusion drive; or you could have a super fast 4TB drive in RAID 0; or a data secure RAID 1 - all software RAID of course. You could also have a super fast, internal Time Machine backup drive. Now, you've got options.
Unless you're going to open up your iMac and make a significant change like swapping out the 3.5" HDD for an SSD, there's no benefit to tweaking around with your existing Fusion Drive because in it's factory default configuration, you're going to get the best performance in terms of speed and usage as well as value for the money possible.