Am I crossing the line copying from the company I recently left?

not a lawyer

Check for non-competition clauses in your employment contract. There can be similar ones there for intellectual property.

You might want to create a solution in the same problem domain attacking the same problem but it might be wise to consider using different mechanisms to achieve the desired outcome (if possible). eg; implement it differently. Use the one you had made as a first-draft.

Another aspect is whether your previous company will value the solution you left with them and if they will develop it in the direction you might have (using the same mechanisms that you would have). If there are differences in values and philosophy the company you left, even with the same starting point, you are unlikely to end up with "the same solution" after a while. Your customers will want different things of your solution than they'd find in the other companys.

In a wider consideration of reality everything is a synthesis of the thing that came before it. eg; "nothing is original". Obviously that's not the opinion of some types of lawyers. The concept of originality exists, but if you can extend the ideas of the previous implementation of the solution and further the novelty, that IP has happened after your era with them. And so capitalizing on that novelty as your edge/point of difference demonstrates you're not trying to copy them, just compete.

It wouldn't necessarily be easy to compete head-to-head with a business that has an existing customer base in the exact space you want to be in. But there is an almost certain chance that they've missed something, made a bad solution to something, are crap at something that you can be better at.

In a litigious society the real answer might come down to who has the better lawyers, but in reality there's no need to have an exact copy of what you left behind. You know more now so you'll make different decisions. and while you're making a new one you'll tailor it to your audience (If you take on board the MVP, indiehackers, lean start-up iterative philosophy).

In my circumstance; I was a data analyst brought on to assist an engineering company writing renewable energy calculators for various uses. They aren't a software house and before I left I asked enough questions to figure if they would value any code contributions I had made. It appeared not. So they don't want to sell software, and are unlikely to value any of the solution(/s) I left with them. So after I departed I re-implemented the solution. Fixed a bunch of things along the way, changed how some parts worked and now it does the job I originally wanted it to do but better.

It is most likely not illegal. However, it burns bridges and may be difficult to compete with. Instead, consider what product you would make if you never worked at your employer given the same problem to solve.

For example, "we need something to help people talk over long distances." Instead of building a telephone, make a two-way radio.

Most companies have a non-compete built into their employee contracts that extends for a year after (sometimes longer). So, you should def check the employment contracts before going forward.

In my (non-lawyer) opinion—even if you’re not bound by a non-compete clause—you should not try to copy your former employer’s product using the knowledge you gained while working for them. Even if it’s not illegal, it’s bad form.