I may have broken ESTA rules without knowing. What to do now?

It sounds like you have only attended the workshop once, so you can tell the CBP officer (if asked) in full honesty that you have no way to compare the current workshop contents with the previous contents. The workshop may have the same title, sure, but coding is still an evolving field. There's more to teach today then there was 5 years ago. So you can't be entirely surprised if the workshop now takes a bit longer.

As Johns-305 points out, you are probably already seen as a low-risk traveller due to your past travel history. As long as there are no new issues they discover in the first few questions, they'll be focussing their attention on higher-risk travellers. And that means they won't spend an extraordinary amount of time on you.

Remember, you're probably seen as a low risk. That's because of two things; not only will they expect there's only a small chance that you suddenly break the rules, but besides probability the other contributor to risk is impact. If you were to break the rules this time, the assumption will be that it would be a minor infraction, not something bad like an indefinite overstay. To summarize bluntly: you're not a political problem.

This may seem like it is legal advice, and I am not a lawyer, but I wouldn’t go volunteering that I had violated the law, without some assurance that I had in fact violated the laws. Such as a judge telling me in open court that I had done so.

To the best of your knowledge at the time it was legal, don’t go second guessing yourself. I don’t see any advantage to you to blurting this out, and if accused of it, would respond that I believe I had complied with the law.

I am married to an American

Immigration hears: My pretense for entering the country is a ruse, actually I plan to violate my ESTA and live here indefinitely with my partner, which is what partners do, because I have zero respect for your immigration laws.

My spouse is American, and I am applying for the correct green card to live in the US with her. Obviously, I don't want to do anything that would put that in jeopardy, and I would appreciate your guidance here.

Immigration hears: I respect the immigration laws of your country, and I am playing a "long game" with the endgame being citizenship. That is more important to me than this trip. I won't to do anything that would endanger the end goals.

As for the symposium, it sounds to me like they deliberately structured it to be "immigration law compliant". They confined teaching activities to just a few hours a day. They made the bulk of it open free-form social and/or homework on their computers, so that time could be productive for you without overrunning the allowable hours/week.

Immigration has admitted thousands of people to similar coursework, so they know more about it than what you told them.

Now, the course managers are finding the constricted teaching hours to be an impediment to their coursework goals, so they are aiming to teach more than 18 hr/wk (3 h/d x 6 days is where that weird number comes from. As such, they are telling you to get a different visa.

Don't worry about the past. In particular, be wary of false guilt or "imposter syndrome".

The rules are:

I could have only attended it if the teaching lessons were less than 18/hours a week.

And:

Technically this was a full-time workshop, but we only got 1-2 hours a day of actual lessons and the rest of the time was spend with the other students to work on projects together.

Not a layer, but I would think that you were compliant with the rules.