It's all good advice. The problem is, I am essentially an introvert and have no meaningful network to fall into.Are you pretty well networked with ex-colleagues, ex-students, parents? If you built good relationships there, I would try to get get in touch with some of those contacts and have them spread the word that you're looking for a new opportunity. Over 20 years, and considering that whoever you talk to will be able to talk to multiple others, you should be able to get a lot of feelers out there.
Esp as we get older/farther along into our career, my experience is that new jobs are landed through contacts rather than applying without a referral or inside connection. Could be a friend of a friend - doesn't have to be direct.
Also, though you may not want to go back into the public K-12 education system, are you open to private schools? Some of the bureaucracy that might be embedded in a public system could be better in a private system where there's more autonomy and if good administrators are there. I only mention that because it's often easiest to get hired into a role very similar to what you have experience in.
Try to remain positive and realize that others are in the same position. I do believe there is age bias against older workers. So difficulty you may be having landing a job isn't necessarily related to you, specifically, but to your demographic group, which is beyond your control. You (and anyone else in your age group) will have to fight harder than someone younger to overcome that bias.
But to reiterate my first point, I would really try and get back in touch with people that you have good relationships with and can be good referrals and, ideally, are also well networked themselves to help you spread the word.
Private schools have their own particular political and funding problems so, no.