Iperf3 is a great tool and is seen as the gold standard in terms of testing network performance however its enormous scope of functionality makes it difficult to configure for specific use cases.
What are the best settings to configure at both client and server end to most closely test general browser performance and specifically offer a reasonably direct comparison to the major browser test sites such as speedtest.com etc?
None of the above, IPerf is not a good simulation to browsing performance.
IPerf generates a (more or less) steady state stream of data, while browsing generates more bursty data with different peaks.
Even for speed testing IPerf is less than adequate, read speedtest test description to understand why- they use an adaptive method and not just "as fast as you can".