I do get the mid-season skip of several years where David goes from lv 1 grunt to lv 99 mafia boss is jarring but I though the Rebecca and David relationship was one of the better developed in the show.

We see early that David is the one that saves Rebecca the most, getting her out of the shootout with the bouncer and also rescuing her when she was held captive by the rival gang. David is also the guy who arguably avenged her brother's killer by shooting him in the head.

We see that Rebecca has interest in David on a romantic level in episode 6 when she invites him dancing and Kiwi basically tells her that she needs to change her approach, showing that this love is one-sided. Rebecca and David are also shown to be really close, Rebecca telling David that she will support him after bringing up the concern that he is using too much cyberware and is at risk of going psycho (in a world where people dont look out for each other as Maine said). She is also the one David trusts regarding the medicine so we see that these two are thick as thieves, especially since Lucy is so aloof.

I had a bigger issue understanding why Lucy liked David to be honest. Its implied she liked him from when they met (showing him the moon landing which is "as romantic as it gets in Night City") but I dont know why given he is just some random street kid. He is impressive using the sandevistan but why fall for David?

Well, I maybe can answer why Lucy fell in love with David, since the beggining David always felt attracted by Lucy, when they first met in the train, he could have just beat her ass without problems but instead of that he work with her, help her to the point of almost fry his brain in the process. She also protect it him, save it and take it to the doctor, to this point and after all what happen David is not corrupted by the city, is even a good kid that always make the best to try to help other over himself, with his mother, with Lucy that barely knew, he don't drink, don't smoke, she felt attracted by the innocence of David and leter we will know why. Then he told the truth about his mother, Lucy is even more related too her, they are more similar that we know to that point. And again, I'm very limited by the spoiler territroy. but yeah, it has some sense.

She was flat out about to kill him though; she had her ghost wire around her neck. She clearly had the upper hand.
I thought the show did try to explain it as her feeling "free" around David. She seems to actually enjoy herself when she's around him and yeah, his "innocence" might have something do with it.

Probably has something do with him being a lost lamb like her too.

As a follow up:

"Cyberpunk: Edgerunners was planned as a standalone work, says CD Projekt Red's Japan country manager Satoru Honma"
gamingbolt.com/cyberpunk-edgerunners-no-current-plans-for-a-second-season-cdpr-says

Hey GOG: If life shelf has been kind of decided, please consider the chance to sell it here on the movies section!
Not the whole world has internet (or a decent bandwidth for streaming) nor subscribed to Netflix...
Potential customers still out there!

The reason why David fell for Lucy is simple, she is the maniac pixie dream girl. A common trope in stories, she is the beautiful, quirky, unique girl that brings David out of the traditional corpo life into the exciting world of edgerunning and shows him the ropes. David pining for Lucy is a very common trope in fiction.

However, Lucy returning that love is off, especially in a world like Cyberpunk where it feels more "real" than fantasy. The story implies that Lucy started to get feelings for David after the first train heist by showing him the vid of a trip to the moon. However, its strongly implied that this was just a ruse to stall David until Maine and Co showed up and she seems pretty disinterested when Maine is planning on killing David and actually smashes his face in. The relatable cause mom died also doesnt make much sense in world because in the CP world, everyone is disposable and David's story is not unique.