I would 100% go for Swift on iOS unless there were known defects in the libraries or the libraries only exist for Objective C. Swift is a fairly nice language and Apple’s multimedia APIs are very sophisticated.
From the devs I’ve talked to, they all say the same thing. Swift is fine for the interface, but isn’t great at the timing/under the hood stuff. I keep debating if I should try and stick my toes in the pool.
From my Perspektive it is important to know that coding is not like learning to ride a bike. You really need to dig in and you need to do it consistently.
Over the last 20 years I also worked with the likes of C++, but I lost most of my knowledge. And its not enough relearning and revisiting stuff in those languages. If you really wanna do stuff in it you have to keep up.
Simpler things its not so bad. You can learn the basics of coding pretty fast and you can a good amount of stuff with that but like everyone before me I can just second for the complex stuff like Audio processing its a long run.
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Depending on the DAW. Reaper and Renoise have built-in scripting, and obviously Ableton has M4L. I want to say Bitwig does too, or at least it has some graphical thing.
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Good UX requires similar latency and performance characteristics to MIDI, so I’m extremely skeptical that Swift can’t handle MIDI. There is no substitute for direct experience, so I would strongly encourage you to give Swift a spin and see if it works for you. It is a very pleasant language to work in and Apple has made it clear that Swift is the way forward.
If you have Ableton Suite then M4L is almost certainly the way to go for prototyping. If you don’t, then python is free. 
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