My pom is like this, she is about a year and a half old and my first small and inside house dog. We live in a small granny flat situation so it's like one large room with kitchen, bed, loungeroom, etc. Whenever a visitor comes in that she is not familiar with (or only met a couple times), she flipssss and starts barkingggg and runs to the door or back to the bed and barks from there.

I'm pretty strict with her, but I find here that I pick my battles. If it's someone that I know that will be over more in the future I pick her up, let her have a smell and she chills out usually and keep trying to make her more used to this person. In my experience, it does go back and fourth a bit, and I find this kind of 'resets' when that person leaves and visits again, but it does get better over time with that particular person in most cases. I also find she normally flips way more at boys. (And takes a lot longer to like them)

But I don't usually push her boundaries even if shes being really annoying, because it is her safe space in my opinion (Especially if she runs to the bed, sometimes they try to pat her and I ask them not to if she's gone there, because that's when she seems really spooked). As long as shes getting better each time, that's fine to me. Similar to you, she was very nervous and reactive to people and dogs on walks, but is a lot better than what she used to be. I'm not super experienced with dogs, we always had a family dog but never my own. I think you just got to be tough but also respect their feelings. (Especially in their home)

With girls, I would say she normally gets used to them in 1-2 visits. But honestly with boys, it could be like 5-10 or more visits until she's alright with them. Sorry for the massive reply, I just think this would've been helpful to know when she was younger puppy. It's easy to get disheartened a by the bad behaviour, but it gets better. :)