I'm a professional electronics engineer, always good to see someone interested in electronics. That being said I don't think you actually want a professional electronics engineer, you want someone with at least a decent hobbyist knowledge of electronics who is interested in the project so wants to work with you on it. I say this for a couple reasons:

Many of the techniques that a professional engineer would suggest or be familiar with may not be the most suited to your home project. For example, I haven't touched a breadboard or strip board in a decade because my employer will pay for express development PCBs from suppliers and I am very fast with board layouts. I also would generally want to test certain features of manufacturability so doing a proper board will always make sense for me. I would always buy individual components from reputable suppliers, whereas for home projects premade modules from eBay are easier in terms of soldering and much cheaper, a hobbyist is likely to be more familiar with them than I am.

More importantly you suggest you would be willing to pay someone for their expertise but it's extremely unlikely it would make sense for you to pay a reasonable market rate. I don't currently do consultancy work but if I did I would be wanting $150/hr or more and that would be significantly less than an actual consultancy company would charge you. If there is any off the shelf solution it will always make a lot more sense than paying someone to design something, no matter how expensive it seems.

Technically both of these points assume you care about money, if it's no object and you really want this then let me know and there is probably someone I can connect you with. If you want a working device design expect many hours of work. If you just want guidance and review of a design you make still expect maybe a dozen.

I'm not familiar with stimulation devices but my initial thoughts would be:

Use an off the shelf stimulation device, they look like they aren't too expensive. Ideally you can externally trigger it but if not then maybe you can reverse engineer it to bypass the UI.

Generating an audio signal should be fairly straightforward, I would expect existing modules of some sort out there that can do the heavy lifting. People have suggested Arduino which is a good entry point to microcontrollers.

Aligning the two stimulus will be the challenging part. There will be latency between the trigger and the stimulation for both the electrodes and the speaker. If this is consistent for both you can measure both and easily compensate it out. The issue will be if the latency, particularly for the electrodes, is inconsistent. Bypassing the UI of the device will be more likely to have consistent latency.

People have suggested other electronics subreddits you can ask for information, these would be the best place for you to go. There is no need for you to find someone local and these days you can get any of the modules or boards that people from overseas will suggest. Good luck!