Yep indeed, that's a tradeoff, if your DB fits into a single volume. I'm not that deeply familiar with databases other than Oracle (that has its own ways to work work around this), so for ease of use, everything on a single volume keeps things simpler.
One thing that I try to achieve anyway, is to spread & "smoothen" the database checkpoint & fsync activity over time via database checkpointing parameters, so you won't have huge "IO storms" every 15 minutes, but just steady writing of dirty buffers going on all the time. So, even if all your files are stored on the same blockdevice, you'll less likely see a case where your WAL writes wait behind 50000 checkpoint write requests issued just before.
well, likely you can just use those as ZFS with ZIL or in more tradional setup with LVM + LVW writeback cache - which from my experience greatly improves latency
One thing that I try to achieve anyway, is to spread & "smoothen" the database checkpoint & fsync activity over time via database checkpointing parameters, so you won't have huge "IO storms" every 15 minutes, but just steady writing of dirty buffers going on all the time. So, even if all your files are stored on the same blockdevice, you'll less likely see a case where your WAL writes wait behind 50000 checkpoint write requests issued just before.