It sounds like you're mixing up SLOG and L2ARC. How is ARC/L2ARC/base dataset not tiered?
Re:An Introduction to ZFS
by hardwaresofton on Friday November 20, 2020 @09:16AM
(#25160594)
ah I did mix this up -- thanks for pointing that out
And yeah, ZFS can't be expanded willy-nilly, found a good blog post with someone's adventures that was illuminating[0].
[0]: https://markmcb.com/2020/01/07/five-years-of-btrfs/