Rationalising buying a new product to get back a screensaver that was working fine on my current device before being remotely removed.... mental
I'm not rationalizing anything. Just pointing out that your comment about a "more expensive" product is incorrect.
This Roku screensaver appears to be the classic "Sachs Marine Aquarium" screensaver that Roku probably licensed, and was also licensed by Microsoft at one time. You can check out a version of it here: https://www.youtube.com/watch?v=9ldeOT3Ysgs
I was able to "cast" this in beautiful resolution to my TV screen on a Chrome browser via my Roku. Of course, you could always just open it up on youtube, lol.
Resourceful people might figure out how to use a USB stick in their Roku for video files they are able to obtain.