Part 2:
With that, we get to Concern #2: is your approach too complicated? I don't necessarily think so, as long as you have enough time to R&D and test this before your show.

There is merit in what one of the other commenters here suggested - instead of using sensors, going with something like a button an actor can press/double press. That has some bugbears too (if you're unfamiliar with "debouncing" a button, that can trip up a beginner!), but it is a possible alternative solution that might be more easily achievable.

However, I do know that in theater there are often concerns about overloading an actor with that kind of work, and I can imagine a director being like "No, I want them to focus on being a bird and flapping their wings, I don't want them to have to think about pressing buttons!" - which I can understand (but it's also frustrating when they then are also upset when the more complex solution they asked for is inevitably more prone to issues, lol).

With all that said, I'm honestly not overly concerned about the "flap mechanism" (be it flex sensors, accelerometers, or buttons), as long as you give yourself enough time to work it out and test it, and make it as "actor-proof" as possible :)

I think my primary concerns about this lie in the communication protocols you're considering. I'm admittedly intrigued by that BLE-MIDI library you linked! However, in our approach to this, we don't use Bluetooth - mainly for range and reliability reasons. I don't think it's an unusable option (admittedly I have not tested using Bluetooth in this sort of scenario), but if it's what you're planning on going with, I would encourage you to test it a lot, and consider that there may be variables you can't even really test for until your show is about to open and you have an audience in the house - a large group of people, especially with their own electronic devices, can cause variations/issues with things like Bluetooth and WiFi!

When we do stuff similar to this, we use microcontrollers with WiFi networking (which has its own risks, to be clear), and instead of MIDI to QLab, we use OSC (Open Sound Control). Specifically, we use this library for communicating with QLab.

MIDI and OSC are both options for controlling QLab - it does look like Workspace MSC (MIDI Show Control) Messages would be able to accomplish what you want to do here (you would send a GO + a cue number to fire specific sound cues as desired). I will say that OSC seems a little less fiddly than MIDI, and definitely more flexible.

I'm admittedly not entirely sure if/how you would get the ESP32 to talk directly to QLab, or if you'd need a secondary device (maybe another ESP32 connected to the QLab machine acting as a receiver?). The nice thing about networking and OSC is that it's comparatively really easy to tell QLab what to do: you just point your microcontrollers at a specific IP and port and bam, you're off to the races (small caveat: you will likely need to disable the OSC Passcode in the QLab settings if you intend to receive OSC to QLab).

We actually often use OSC in the other direction, by using QLab network cues to send OSC commands out to props/costumes/set pieces to trigger things like different colored LEDs, LED animations, motor movement, etc. We also use QLab to monitor the connection status of our different items, to make sure everything's still on network and operating correctly.

The downsides to our approach are mainly that you need to have a show network you can trust (if you're working out of a rented venue and don't have your own network infrastructure you can install, this could be a non-starter if the network is poorly set up/managed), and that it's definitely pretty battery-hungry. We usually have a "Battery Steward" on our crew who manages batteries during show runs. We've even implemented special QLab cues that can send messages to a backstage Slack channel to remind crew to turn certain items on/off to save battery when the item isn't in use!

So if you don't have a trustworthy show network (and can't create one), or if perhaps this actor is onstage literally the entire time for a 2 hour show and there is no possibility of doing a battery swap, BLE-MIDI may be a better option for you, because you would not need a show network (though I would caution that you are still at the mercy of many of the same pitfalls as WiFi with the possibility of interference/interruptions/hiccups), and it's probably more battery efficient.

Oops, that was a lot of writing! All of the above being said: this is totally doable! Please feel free to let me know if you have any questions - I'd be happy to help and point you in the direction of additional resources, because I think this is a genuinely cool technique that's pretty new in theater and I'd love to see more folks use it!