At the current pace, it won't be until 2024 or possibly 2025 that someone can sit down and purchase the RULES to play RMU. Since they "finished" the 1st book, RMU Core, it has been 5 or 6 months and there is not even a timeframe for the 2nd book, RMU Spell Law. I dont imagine there will be a speedier process for release of the subsequent and necessary rule books.
Yes, having the rule books out should take precedence over putting out the source and setting material, BUT, sadly, with the glacial pace at which they put out any material, it could reasonably be 2026-2027 before they get anything out for their core setting. It could be hyperbole but it seems like it might be more likely that my dates are closer to the truth than what they will admit.