I think I've made up my partitioning plan:
SSD [128GB]:
HDD [3TB]:
- /data [2.8TB] (symlinked to /home/data)
- /swap [8GB]
- Leave the remaining ~0.2TB for safety reasons - e.g. if at some point I'd want to create Windows partitions
I'll also move /tmp to RAM (which I'll upgrade to at least 8GB).
Thanks for all the answers.