from my experience the lighting guys either use crappy/crashy opensource GUI apps or propper lighting desks for several thousand bucks. Everything else is DIY with computers where you get a little hacky. The only out of the box solution I found right now is this: http://www.raspberrypi-dmx.org/raspberry-pi-midi-dmx-bridge … but it seems there is no documentation. I guess the easiest way would be indeed as suggested to use something like PureData. Unfortunately openlightingarchitecture OLA is not having a MiDi plugin.
There is also an iOS app called Luminair …but it’s 110€… and you’d have to get your MiDi into your network first and the DMX out of the iPad again (…that’s where ArtNet is handy).
Then if you want to make visuals anyway you might want to have a look at VDMX (macOS only) or MadMapper (win+macOS) which have both MiDi + DMX implemented. Best way to use this is to have an artnet node (ArtNet is DMX over ethernet) - so a raspberry pi with DMX out and ArtNet in or just use the Raspberry Pi directly with a few LED strips. This is not an easy topic… I tried to dive into this topic but the options out there are not too overwhelming but I might have missed something.
Just a few thoughts… maybe it helps.