I have sympathy for your situation.
It seems that data on the emotional impact of being a sperm doner child is not greatly studied. However, just imagine the emotional impact it might have to know that your biological father cared so little about you and his genetic material that he was absolutely fine with you being raised by a total stranger he'd never met. In fact, they knowingly gave it away being happy that they'd never be involved in their own child's life.
It's easy enough to wave this away, but parents cannot nor should not gaslight a child into believing that this doesn't matter and that your their love & care is enough.
It's not just a question of ethics though. One massive thing that women learn the hard way is that you cannot eradicate certain personality traits with the right upbringing alone. Your child might be just like you, or a mix, or they might turn out to be exactly like their biological father, which may be a very bad outcome.
You can't predict that as you won't even know the father, and there is a myriad of very unpleasant character traits you cannot screen out via background data alone. Plenty of horrible people never get criminal records. Psychopathy & narcissism, (both inheritable traits) is more highly represented amongst certain professions (surgeons being one of them). Now imagine coping with that in a child or teenage boy when you're a single, female parent.
It's out of concern for you and women like you that I point this out. I know someone who's decided not to have a 2nd child, mainly because her boy had been such a handful and it's clear he's inherited most of his personality from his father (which she knows for sure as the child wasn't a doner child). I think on some level, she's concerned that her 2nd child wouldn't be safe with him...and he's under 10 years old currently so who knows what he'll be like later in life.
I really feel for you. It's really not what you or any woman would have imagined for herself and I understand why you want to feel more like a family unit rather than just you and your current child. There's social benefits to having siblings too. I don't know what I would do to be honest, because on the other hand, it seems there's really strong data to suggest that a child is at most risk of being abused, either sexually or otherwise, by a step-father, so it's not risk-free to your current child.