Screenwriter Park Hye Ryun is responsible for 3 dramas on my all time top 10 list. Because of that even though Start-Up was a huge disappointment I've let the comment section's hype get to me.
What's good news I suppose that this is far far better than Start-Up. (Which I may have dropped half way or so in, I'm not even sure anymore)
Anyhow the plots and themes in Castaway Diva are very much Park Hye Ryun-like, despite the fact that she is not the only writer, probably the other writer Eun Yeol contributed more to dialog, I don't know. Anyway anyone who has watched the big three dramas I mentioned before (While You Were Sleeping, Pinocchio, I Hear Your Voice) will find many subplots & the general direction of the story in this drama oddly familiar. Near the finale there is a pretty iconic scene that is 1:1 While You Were Sleeping as well. Not that I mind this too much, Park Hye Ryun always heavily built and iterated from her previous works in all her dramas and I absolutely adored them..
So you might ask, why did I rate the writing at 5.5? Well, out of all the preproduced (non live filmed) dramas this was perhaps the most inconsistent (that I can recall anyway). To me it seems like the writers shaped the plot assuming that the general audience won't remember the minutiae of what they've seen 2-4-6 weeks ago. Unluckily for me I binge watched this over the course of 4 days during long daily workout sessions, as such in almost every episode I had several moments where I was rolling my eyes at all the gaslighting that has been presented to me. Also even if I ignore the plot itself and look at the various world building elements, the drama is -even by kdrama romcom standards- just incredibly poorly researched. Pretty much every little detail about the Island was wrong. I understand this is fiction, but it would not have been very hard to ask someone well versed in wilderness survival to give the writers some tips & to use common sense. (I could list countless things here, but just to say a few: potatoes, FL's well groomed-ness, swimming in baggy sweatpants/shirts, the shoes)
The biggest writing related pain point though is that this is like a romcom with most of the romance ripped out of it, but the rest of the things are left as they would be in a really silly drama about a cat shapeshifter or something else like that.
Anyway I did not really feel the leads bonding at any point in time, as opposed to the second lead doing some of that. Basically the love triangle here is very similar to what we've seen in While You Were Sleeping, but with the amazing main romance missing, if that even makes sense xD (meaning the second ML is not a nuisance, it's just this is so weird overall)
If you are curious what a good romance-less romance drama is like, take a look at the cdrama Reset.

Onto the second big problem I had with the drama, which is Park Eun Bin, or more specifically her portrayal of castaway Seo Mok Ha. I have seen this actress before shine in Age of Youth. (I don't think she is lacking in skill, in fact she is probably awesome.) Also while in Attorney Woo I very strongly disliked how she acted as an autistic lawyer. There she did the usual Hallyu version of "person struggling with disability" act and probably did it as well as it is possible, but I always found that incredibly offensive in any drama, but I digress. The point I'm trying to make is that someone had the genius idea to not only cast her for this role, but also make this castaway character's portrayal essentially the same, or maybe because this was her next drama after Woo she did not completely recover from that role which she has taken way more seriously as an actor. I suppose instead of all this ranting I could have just said that she is overacting in this one, but it just feels wrong & weird.

The casting in general is not all that great. I love watching good sismances in dramas, but I did not feel that sister chemistry all that much, though all the back and forth in the plot did not help. Nor did FL have good chemistry with ML, but there was barely any time to build on that, so I could be wrong. Though I must admit the evil dads were quite fit for their roles, despite not having seen these supporting actors in evil roles thus far.

Remember how the final 3-4 episodes of Pinocchio were after due to it being a huge success they extended the run from 16 to 20 episodes? Walkaway Diva has ~8 episodes like that, out of 12 which is obviously not great. Earlier I ranted about the writers expecting the audience to forget stuff, now that I think about it, perhaps that could have totally worked on me too, because I don't think I'll remember much of anything of what happened between ep2 and 10 in a couple of weeks.

I apologize for all the rantish walls of text so far, I too am getting tired of writing this review, so I'll quickly list -in no particular order- what I actually liked about the drama:

⌲ Episode 1 is a great short film, with very talented child actors [10/10]
⌲ Everyone is really attractive, with very pleasant, almost hypnotic voices.
⌲ The entertainment industry related procedural parts are usually far far worse. (they avoided many common themes that tend to ruin things in other dramas about idols)
⌲ The parents -adoptive dad+mom- were adorable.
⌲ The OST and all the singing is really really and I mean REALLY good. (though it's annoying that everything is basically lip syncing, because all songs are perfect studio recordings, where there was no effort made to make them live-ish during editing)
⌲ There are no significant misunderstandings.
⌲ It's a high quality production with great photography & across the board great audiovisuals.
⌲ Evil Dad #2 was a relatively high class kdrama villain & did not overstay his welcome. (similar to other Park Hye Ryun villains)
⌲ There were some very satisfying -power-fantasy like- scenes.
⌲ The dialog has some rather great moments, here and there.