Thanks for the suggestions. strace did reveal that cp & rsync were sending the 128k & 256k writes. rsync also had some select blocking which could explain the increased copy time even with the bigger write size.
dd is sending the expected read and write requests based on the params specified so this is ideal for more testing now.
Unfortunately, however, I didn't notice any significant different in the write speeds with & without max-pages on both rclone & mergerfs, however, mergerfs was showing almost 2x the speeds of rclone (and could probably go even faster, since my drive write throughput got maxed out at those speeds).
Reads seemed to be similar with & without max-pages in mergerfs but this is probably because my drive read throughput is maxed out at those speeds.
rclone was slightly slower than mergerfs but still had a 3x-4x throughtput increase than without the max-pages.