There is a thread in the forums about coming updates for the RM mixers. Midi is on the way, but it is probably not part of the update today. Here is the link: https://forums.presonus.com/viewtopic.php?f=222&t=23012.
I think you need to understand that the SL III mixers & ecosystem are like a counterpart to the RM mixers & ecosystem, which basically is the CS controller, while the SL III will have 5 stageboxes and a "personal monitoring system". We need to consider them as two different "families" of products, which are not compatible because of different chipsets and therefore different "evolution stages" of AVB. PreSonus is still trying to somehow build a bridge between AVB I and AVB II, but obviously things do not look good. This simply means that we should not count on a future solution, but plan and work with the tools available.
And you need to understand that the SL III mixers are fully based on AVB network solutions, so the "stageboxes" will be part of a wired network (CAT5/CAT6, AVB switch, router). Of course there will be remote control, but only for the mixer. The stageboxes have no processing, at least not for mixing. On the other hand in a combination of CS18 and RM the processing is done ONLY on the RM mixer. The CS18 is just a "stupid" controller, but in an AVB network it can also handle 4 input and 2 output additional channels of audio. In a direct wired connection CS to RM or in a Dante network the CS18 can be used only as controller.
If a series III mixer will help you in your studio work, get one - and use the RM + router + iPad for concerts. Or sell the RM mixer, get a SL III and add from the SL III ecosystem what you need when it's available later this year. I think the "personal monitor mixers" will make the SL III series very useful, especially if you work in both worlds (studio & live).
To be clear: The SL III is a mixer with much more processing power than a RM mixer has. The CS does not add more power or functionality to the RM mixer. There is no plan and no possibilty to add the new SL III features to the AI mixers.