Which licence do I choose, publishing a previously closed-source product?

Question

From 2001 thru 2019, a company "A" that I own and I am the CEO of with HQ in EU, had a commercial software product for sale on the international market. We issued perpetual per-site licences to what are mainly large industry clients all over the world, prices depending on client size and modules used, ranging from roughly USD 5,000 up to USD 500,000 to pay once, and made a living off that, and from maintenance contracts which were yearly ~17% of the licence price, plus the occasional bespoke add-on project from time to time.

Then a partner from another EU country "B" ripped me off, conned me, took advantage of me, did me up like a kipper. Formally I am still owner and CEO, but they managed to seize the company and deprive me of my estate. We had an escrow contract which would allow "B" to obtain the source code of said product for EUR 200,000 but only in case "A" went out of business, which it didn't. Some lawsuits are in preparation, some are ongoing already. I have seen not a cent so far. They continue to sell my product as their own now, and keep the money.

My lawyers support the view that I am the de facto owner of "A" still, and "B" might have committed punishable offences in the process. Also, I am personally the developer and author of > 90 % of said source code. The rest was written by my "A" employees.

I plan to publish the complete source code along with documentation and build instructions on github. The intent is to give it away for free, render it worthless for "B", enable existing customers to terminate the contract with the "B" company (so they wish), and offer them, as well as prospective new customers my services for additional development, maintenance and support.

Which license model should I select, to best match this intent?
What do I write to document the transition from closed source to open source?

I don't ask for legal advice regarding the status of my company ownership or overall legal strategy, details of which I mentioned for context. Let's focus just on the licensing question.

Answer 1

I plan to publish the complete source code along with documentation and build instructions on github. The intent is to give it away for free [...] and offer [existing users] as well as prospective new customers my services for additional development, maintenance and support.

Which license model should I select, to best match this intent?

Any free licence will work with that model.

However, if you were to release under (say) a permissive free licence such as 3-clause BSD, other people (and here I specifically mean "Company B") could take the improvements you publish and incorporate them back into their closed codebase.

Assuming you don't want that to happen, then if you were to publish under a strong copyleft licence, say GPLv3, others could only incorporate those changes if they opened up their entire codebase to their customers. GPLv3 ss 5c, 6 require that modified works (and the entire codebase becomes a modified version of your patch, once they incorporate it) be available, in full source code form under GPLv3, to anyone who has a copy of the binaries.

This doesn't, however, deal with SaaS customers. For maximum protection from closed-source competitors absorbing your improvements, consider Affero GPLv3. This functions similarly to GPLv3 but extends the right to receive AGPLv3-licensed source to people who interact with the work over a computer network. It seems to me that licensing under AGPLv3 will prevent your competitor from incorporating any of your changes into their offering, whether provided on-premises or as SaaS, unless they are willing to open their own product up as you intend to open yours.

And of course, IANAL/IANYL. You should take specialist legal advice from someone who understands free software licensing before you bet a company on this strategy.