Rich,
Thanks for posting your question. I know how you feel. My last job was killing me. I work in state government. Last year, I found a better position with a different state agency. Night and day. Transferring was easy, since it's the same employer essentially. My health has improved. I now actually sleep at night. Incredible.
Since you should work a few more years, find a better job. Once you start looking, you will start feeling more positive about life. Make that your priority. Engineering work is everywhere. There must be something closer to home, maybe with better pay and better vibe for you. Also start getting your money socked away in instruments that will earn more. Read every thread here about investing, the index funds, all the tax-free options, etc. Amp up your savings immediately and put everything you can in those funds. Since your SO is onboard, you should both be getting as much money as you can into the best investments you can find. SO may not hate the job now, but we can all come upon circumstances at any time that will prevent us from working.
Like another poster said, start the side hustle. Everything you do outside of the hated day job will lift your spirits and you will feel less trapped. Get a few students signed up and start building that reputation. Figure out what you need to do to advertise and who you need to meet (music teachers at middle/high schools, music store employees, and so on). Figure out what others are charging. Make a teaching video and post it to YouTube with a mini lesson as advertisement. Find a coffee shop where you can perform and hand out business cards.
Test the waters while you keep the job and get a handle on the money. Life will get better. You should start a journal here. I'd read it.
NDQ