A very clever spin on blackjack. It keeps the basic concept of knowing when to stop, and the excitement of pushing your luck for more cards, which helps it keep the spirit of blackjack while expanding it in an interesting way. The one thing it's arguably missing from blackjack is that every single hit is exciting and ultimately matters a lot, whereas naturally if you expand the card count by a lot, you do lose a little bit of that excitement for each card. Though I don't think that's inherently a problem, it might be worth considering how you can play up those moments that you have to draw a dangerous card, or potentially using those dangerous moments to tempt the player with an especially dangerous choice. I think there's an opportunity to do something cool on the "exciting draw" front.
I felt Aces were a little bit of a weak spot in this, for a few reasons. One, it was difficult to really know where I stood on my number, as it's difficult to keep track of which of your Aces are 11s and which ones are 1s. Might be useful to highlight them or something while they're 11 to dictate when that safety net exists. Though it's worth noting if you have enough 11-point aces, it does make your hand much more flexible and removes a lot of the risk.
I did feel that my final build made the game extremely easy. Specifically, I had half cost positive, half cost negative, no face cards, 25 value max, and 25% bonus; with that I was able to draw to my limit 90% of the time without ever risking a bust (with an average of about 2000 points per hand). Nerfing the aces a bit might help, maybe limiting upgrades too so that they only work on either positive or negative cards/not being able to get the upgrade for both sides. Raising the value from 21 to 25 also felt very powerful, since it raises the ceiling it gives you an average of +4 per card and greatly increases your safety threshold (though it's possible those benefits were exaggerated due to my half point upgrades).
I was a little disappointed that cashing in on blackjack didn't give a bonus. It's an extra-risky value to go for, and it could be fun to have to decide whether to cash in for the perfect number bonus, vs trying to go for more cards.
But to reiterate, I think this is an excellent and very well designed game. I had a lot of fun with it.