From my POV now, the i3 instances should never be used unless you absolutely need the dedicated local storage in those amounts. The per vCPU performance is horrid in comparison to any of the modern instances (R5/M5/C5) let alone comparing those vCPUs to the actual CPU of R6g/M6g/C6g like you said.

Another advantage of the Graviton processors is 50% more dedicated storage compared to equivalent Intel/AMD instances. To get enough storage we would have had to bump up to r5d/r5ad.24x or metal which when testing we also saw more “jitters” in latency on the long tail. Despite the x86 instances being larger they traded blows in different tests we had, aggregates were one thing that x86 easily beat out ARM but a lot of our aggregates come from another data source so it wasn’t a deal breaker. Overall we are happy with performance, compared to old stack we are at around 10% of the cost and I think our savings was more than 2x compared to x86 after locking in some rates. R6gd.metal (16x) vs R5d.metal (24x)