I’m a parent of a teen with extensive mental health issues and agree with many of the suggestions that have been made and affirmed above. From my own experience, I will add this:
Don’t take the phone away unless they are doing dangerous things with it (posting/sending inappropriate photos, communicating with older people seeking illegal things, etc.). My kiddo uses their phone as an escape and distraction from anxiety and it actually is a helpful tool. If they have social anxiety, communicating via text, socials, etc. is a great, controlled way to keep up with friends and feel close when depression and social anxiety might make face to face interactions hard.
When you are depressed or have anxiety, having control and agency over your life is important and taking things away can make someone feel less in control. Dangling rewards works for some kids and not others. I could offer my kiddo a million dollars to see their Psychiatrist or Therapist but if they don’t want to go, they won’t go.
You can go to a therapist and should. They have a lot of helpful tips on how to approach things with your kids, approach difficult conversations, let you know if you are handling the situation correctly, what red flags to look for, and helpful resources in your community.
Your kid sounds smart and well read. Point them to some online mental health resources that they might read and engage in. They are some great phone apps too.
Use that vaccine appointment with their PCP to also have them screen for depression and anxiety and have their doc make appropriate referrals to a therapist or write a script if needed. Sometimes a kid with depression needs meds to decrease their symptoms enough to reduce the fog around them enough to make them motivated and willing to engage in therapy and other activities to gain a skill set to manage symptoms. That in turn can sometimes reduce/eliminate the need for meds.
Let them take the lead with senior year classes and collège. Definitely open the door for them to modify their senior year schedule and introduce the idea of attending a CC or 4 year school nearby and living at home. As a possible poly sci major, maybe they want to take a gap year and work the campaign trail for 2024?
Do fun things together and try to connect. Even if it is just snuggling in their bed and watching a TV series together. Provide lots of healthy meals and snacks and opportunities for hikes, biking, etc. Exercise and healthy eating can help ease depressive symptoms.
Best of luck to you and your kiddo. You are already a great, caring parent.