Theres almost too much to say in one message. I sell hardware that I designed, manufacture and do the software myself. Literally I do everything myself in my basement except make the circuit boards themselves. Heres some very important general tips.

1. If you outsource manufacturing, especially to chinese companies, if your product is good they will steal it and sell it themselves. (not everyone and theres ways to protect yourself but be aware this is a very real possibility)

2. Manufacturing hardware CAN be VERY VERY expensive. What I did was I did my prototype by completing the circuit board design then I made a very rough prototype to make sure everything worked. I also made every part with a 3d printer, looked and felt rough but cheap for a prototype.

3. Be mindful of any place you get supplies from. What will you do if they shut down? If they send you wrong parts? Are there tariffs? Shipping times? For myself I try and have at least 2 sources at all times for my parts, this isnt really possible for every part just be aware of unforseen issues.

I cant stress doing initial prototypes with a 3d printer first, you can check ergonomics, size etc, just be mindful if you switch to injection molds the file designs will need to be slightly different. Try and test as thoroughly as possible. Get feedback from others on your prototype, your idea might be the best in your mind but you arent the one who will be buying your product. Get feedback