I've basically gave up and went for a RAID0 configuration at the end. Some resources I've went through, just when someone cares about:
- https://livevirtually.net/2014/06/27/configuring-hp-smart-array-p420i-io-controller-for-vsan/
- https://www.ixsystems.com/community/threads/freenas-only-detecting-2-drives-of-smart-array-p420i.69973/page-2#post-48688…
- VSAN Mixing SAS / SATA Drives with HPE P420i - illumo
@RattleZ: I'm now running the P420i in RAID0, with 1 GB FBWC and battery attached. SSD SmartPath disabled (as recommended in 1st blog article above). The queue depth seems to be 1024 on my machines.