After a bit more research, I’ll endeavour to (tentatively) answer my own questions (since nobody else has offered any info), just in case it might help other people looking for the same info:
I suspect R and S won’t be released in stable format for a while. It seems /e/OS devs are very thorough testers so they are not eager to mark a half baked distro/release as “stable” (which is a good thing). I also suspect R and/or S won’t be released as stable until Murena is ready to release new phone(s) sporting each of these /e/OS versions.
Since /e/OS is a forked version of LineageOS, it cannot really support hardware not supported by LineageOS, and by the looks of things, LineageOS has no immediate plans to add newer Samsung models to their support lineup (I suspect it’s a huge amount of work to prepare and test drivers and hardware functionality for a new hardware model)
As an additional note, after extensive research (and reading between the lines), my educated guess is that LineageOS does not intend (at least for the foreseeable future) to end up supporting the latest phone model from any/most phone manufacturers.
*** please take all the above “with a grain of salt”, as it is my own opinion/speculation and not hard facts ***