Any downsides in terms of my biological father

Excellent question — you're thinking this through carefully.

From a legal relationship standpoint:

Once the adoption is finalized, your biological father is no longer your legal parent. This means:

• No automatic inheritance rights from him — you'd no longer inherit from him automatically if he passes away without a will (though he could still name you in a will if he chooses)
• He loses legal standing — he would no longer have automatic legal rights to make medical decisions for you in an emergency, access your medical records, etc. (though as an adult, you control this anyway)
• Legal ties are severed — in the eyes of the law, your stepdad becomes your parent and your biological father does not hold that legal status

However:

• Your personal relationship can continue — nothing stops you from maintaining whatever relationship (or lack thereof) you currently have with your biological father. The adoption doesn't prevent you from having contact with him if you choose to in the future
• He wouldn't necessarily know — since he doesn't need to be notified in most states for adult adoption, he might never know about it unless you tell him
• It's your decision — you're an adult making a choice about your own family structure

The real question is: Do you want your stepdad to be your legal father? If the answer is yes — if he's been the father figure in your life and you want to formalize that — then the fact that it severs legal ties with your biological father is kind of the point.

Many people in your situation see it as finally making the legal reality match the actual reality of who raised them and who they consider their parent.

Does that help clarify things? Are there specific concerns about your biological father's reaction or future relationship that you're weighing?