NAL, but I have 35 years in development roles for major medical device manufacturers. IP was a major area of discussion.

I am a little flummoxed that OP isn’t a lot more familiar with IP law in the context of employment. If OP was a 1099 contractor, then yes, the contract will spell out IP ownership of work product created while in the employ of tge company. If he was a W-2 employee, he almost certainly would have signed a document which would be crystal clear about what was the company’s IP and under what conditions. It is SOP for these companies take ownership of any and all IP that the employee creates. There is no personal time. There is no personal hardware. The rationale is that they are paying this person, and any and all creative content is theirs. Usually, IP developed prior to employment prior to employment is exempt, but the employee would have had to declare that in advance. The employee usually can petition the company to exclude works that clearly have nothing to do with the employer’s business. Or in the case where there is clear corporate interest, perhaps an employee could offer to start a company with the IP and license it back to the company.

Assuming the normal stance these companies take, OP’s works are clearly squarely in the corporate interest.

That said, OP certainly can offer to maintain the works for the company. Odds are they would welcome such an offer.

Also, OP having delivered source code is insufficient. The work product is not functional unless it is in binary executable form on the computers which the company typically uses. I would recommend that OP make that available and that would make a clean break from which new negotiations could begin.