General answers
SLOG
Rule 0: You probably do not need a SLOG.
Rule 1: A SLOG should have PLP, high endurance and low write latency.
Rule 2: Size should hold two transaction groups, so 2*5 s by default. (Larger does not hurt, save for the wallet.)
16 GB M10 Optane fall somewhat short on endurance and throughput for SLOG; however they make nice boot devices. Although the size of a M10 would be sufficient, the real entry point for an Optane SLOG is a 100 GB DC P4800X.
L2ARC
Rule 0: You probably do not need a L2ARC.
Rule 1: Bump RAM first (64 GB minimum). Then run arc_summary and refer to Rule 0.
Rule 2: Between 5*RAM and 10*RAM. No more!
Mind the gentle Dragons!
For the adventurous homelaber, a single Optane 900p may be partitioned into SLOG and L2ARC.