A while ago I needed to briefly retain a lawyer to navigate a complex contract with my employer, there may be terrible lawyers out there but the person I reached out to was exceedingly thorough in explaining things (and I, as a dev, had lots of rules-lawyery questions to throw at them), quite prompt in their response and charged a modest fee.
Speaking to a lawyer will absolutely cost you more than a coffee, but it's not that pricey in the scheme of things compared to getting in legal hot water.
Without a writ, you have little idea what the substance of their legal claim is; there's no purpose in hiring an attorney, because there's nothing for him to work on. So you are saying: "I don't fold to hollow threats and bullying. If you have a proper legal claim, make a proper legal claim."
If you then get a writ, show it to a lawyer.
If you have no substantial assets, it might be worth pointing that out (once the writ arrives). Nobody wants to sue a man of straw.
I do agree with those who say get a lawyer to send your response. However you should still do the research yourself - to the best of your ability. You can save yourself some expensive lawyer time by knowing the basics and having a potential draft (be careful: make it clear that you do not have any emotional ties to it so the lawyer is free to tear it up) of your response.
If it goes to a legal proceeding, it is about convincing a stranger and lay-person (i.e. Judge, or perhaps even worse a group of jurors), that you didn't violate the law. You have to do this while the other side is doing their very best to convince the same stranger that you are a devious thief who did irreparable harm to their company and cost them millions in damages (or some other absurdly large number). They will paint everything you did and say as part of your plan to steal from and damage their company. They will have highly qualified experts submit very convincing reports, and testify, that what you did was trade secrete theft, and caused immense damage. Your only defense will be to push your own narrative that can counter all of this, sufficiently to get you off the hook. You'll need at least a lawyer, perhaps experts of your own, and all the cost that this entails. This will be a big deal to your life, but will be a business expense (i.e. before taxes) for the company suing you.