I could be wrong about this, but I seem to recall reading at some point (prior to the release of the first coin) that the 'Rwanda Nautical Ounce' series would be limited to a four-coin series only (both in BU and Proof versions):
2017: Santa Maria
2018: HMS Endeavour
So, much as I've liked all of these designs to date, I wasn't actually expecting any further releases.
The 1oz and 3oz antique high relief versions of the Mayflower only, which I agree are very impressive, were intended as a kind of artistic 'final flourish' to coincide with the ship's 400th anniversary in 2020, I believe.
Having said that, I wouldn't say no to another release, if it does actually happen!