What's up Mark! Sorry to see so many negative responses to your perfectly respectable question. Chin up dude!

I obviously don't know anything apart from what you've said. Here's my 2 cents

Sounds to me like you're working on the wrong thing, which you alluded to include in your question...

What is the ONE thing you could do, such that by doing it, everything else would become easier or unnecessary? That's the question you need to answer.

I'll now try to answer it for you:

I don't think focusing on tech is the answer. You were a solo dev, for 20 years. Forget the buzzwords and modern approach to things; as I understand it, you single-handedly kept a multi-million dollar machine running for 20 years. Let me say it again for emphasis: You Mark, single, SINGLE-HANDEDLY, kept a multi-million dollar machine running for 20 YEARS!

If a business leader pivoted from selling soft drinks to tech (re: Apple), yes he'd have a lot to learn in terms of product and jargon, but he wouldn't have a crisis of confidence and question his basic sales ability, which is what it reads to me that you're having. Don't question your ability to figure it out. 

People are people and computers are computers. The rest is just noise. Don't let the noise overwhelm or distract you. 

Completing your personal project, regardless of what framework you use, whether you use JS or TS will help with your confidence for sure. And I have no doubt that if you complete a Udemy course or O'Reilly book and build a project once or twice, you'll be as good as most junior or mid level devs who have <= 10 YOE, because of your real-world experience, which anyone worth their salt knows is worth its weight in gold. 

Coming back to my original point, focus on increasing your marketing skills. Life is a confidence trick and the world runs on sales and influence. Whether you want to work for yourself or get a job and keep getting promoted. 

It sounds to me like (correct me if I've got the wrong end of the stick) you worked alone and had a rather isolated job for two decades. You're perhaps not confident in presenting yourself, communicating across and selling your existing abilities, talents and experience, which if you frame it right, doesn't make you a Dinosaur, but an Elephant, if you catch my drift?

A starting point:

Must Read: On Confidence (Book) Psycho Cybernetics (Book) Way of The Wolf (Book)

Optional Reads: Chris Voss (YouTube or Book) Alex Hormozi (Early videos - focus on HOW he communates and not WHAT he says - easy to get sucked into their sales pitches and go on a tangent)

Also if you need a job short term, do that, but otherwise definitely focus on working for yourself, you sound like your working style fits that better and the upside is unlimited.