With actual RS-232 buffering a simple daisychain connection should work just fine at that baud rate. The limits cited for RS-232 are typically 50 feet or a max cable capacitance of 2500pF for rates up to 20kbit/s. Longer distances or higher rates are possible with low-capacitance cables.
Now, with the 3.3V TTY link you're proposing? Not so much. The noise margin of such an approach is suspect at best; you have multiple boards to deal with so ground loop issues can be a problem. If you insist, you should consider rebuffering the signal.
Your intuition to use RS-485 is a good one. It would solve those problems and help future-proof your design. RS-485 avoids a number of issues that RS-232 has, such as noise immunity and more-limited speed at distance. Since you’re already using RJ12 you have enough pins, so might as well go with that. And it can share the 3.3V supply with the rest of your logic.
Oh, and you can choose RS-485 that doesn't load the line down if the module power is off. This isn't so straightforward with logic-level interfacing, though it can be done.
There is a popular protocol used for theatrical lighting called DMX512, that functions exactly the same as what you’re building. It’s a multi drop serial line, and each client is addressable. DMX512 uses RS-485, with up to 64 clients per loop and up to 400m in length running at 250 kbit/s.
More about DMX512 here: https://learn.sparkfun.com/tutorials/introduction-to-dmx/all
At the risk of killing your enthusiasm, are you possibly reinventing the wheel with your MIDI-to-solenoid idea?
MIDI to DMX: https://www.instructables.com/id/MIDI2DMX/
DMX solenoid driver: https://www.amazon.com/Switch-Dmx512-Controller-Output-Control/dp/B00S9KABRA/ref=asc_df_B00S9KABRA/