Ok, I have answered my own question with a bit more (unscientific & uncontrolled) testing
My sample was 5 test files, all exactly 1,000,000 KB each, or 4.78GB total (according to rclone)
rclone --transfers 1 = 4hr, 35m ~333 KB/s
rclone --transfers 1 --concurrent-uploads 4 = 2hr, 16m ~700 KB/s
rclone --transfers 1 --concurrent-uploads 4 --s3-chunk-size 16M = 2hr, 21m ~680 KB/s
rclone --transfers 1 --concurrent-uploads 16 --s3-chunk-size 16M = 0hr, 37m ~2450 KB/s
rclone --transfers 4 --concurrent-uploads 16 --s3-chunk-size 16M = 0hr, 15m ~6000 KB/s
So, multiple transfers with multiple uploads works better for large files to Wasabi. Chunk size didn’t make much of a difference.
Interesting that my last test actually completed the first 4 of 5 files at 100% in 5 minutes, but rclone took a further 2/3 minutes confirming the upload before moving onto file 5, which, as the last file, took another 4 minutes to reach 100% and another 3 minutes to confirm, before rclone returned to the command prompt.