Raid is not backup. This is not only true but with Raid on conventional filesystem a daily necessity as every power outage may produce a corrupted raid or filesystem. Any virus or accidental delete require the backup. So backup is part of daily work.
ZFS ist not backup. This is true but only in case of a real disaster like sabotage, fire, theft or a flash. ZFS is intended not to loose data during regular operation or virus/Locky attacks due its CopyOnWrite (crash resistent, no corrupted filesystem or Raid per design) or versioning with snaps. I do backups to two backup systems, one in another building but I have not needed the backups since I use ZFS. Access to Snaps is the daily recovery option. So backup with ZFS is disaster backup.
about number of disks ver vdev
ZFS does not care if you use a Raid-Z1 with 2 or 20 disks. The rule that you should data disks per vdev as a power of two (2,4,8,16) + redundancy disks is more duer the idea that you write datablocks of 128k, 64k, 32k etc. With a different number, you have always a small waste with a slightly reduced capacity compared to what you bought.
Performance wise, the number of vdevs is not relevant regarding sequential performance. This scale with number of data disks only. But as you must position each disk to write or read a datablock, the iops of a vdev is like a single disk so the more vdevs the higher the iops. This is the reason for massive n x raid-10 configs in the past. Today you use SSD if you need iops.
about Slog in an SSD only pool
If you enable sync, you must write every datablock twice, once immediatly as a slow small write and once as a large fast sequential write over the write cache. If your Slog is not substantially faster (lower latency, write iops) than your pool, an slog is useless. Only one problem remains. If you use an SSD pool without powerloss protection, you should always add a fast Slog SSD or NVMe like an Intel P750/3600/3700 with powerloss protection. But for critical data you should always use pool SSDs with powerloss protection to allow background garbage collection without data risk in case of a power outage.