Hi
Kari2017, great question, and I'm curious to see what others here suggest -- the community here is a great source of wisdom.
To build the fluency required for rapid accuracy, I recommend as much active learning as possible -- deliberate practice and quizzing yourself -- instead of too much passive learning. The research is clear that the best way to build skill is active quizzing, and lots of review & re-dos based on your errors (along with coaching on strategy, applications, and avoiding traps).
MGMAT Foundations of Math is a good place to start for Quant -- you wisely understand that it's essential to start with the foundations.
https://www.amazon.com/gp/product/B07P5HFN7HMGMAT also has an app with hundreds of foundation drills (screenshot attached), in an efficient format that allows you to track your confidence on each question, but it doesn't include structured reading, as the book has. Some are free, but most require a premium upgrade.
Web app:
https://gmat.builtbyhlt.com/Android and IOS apps:
https://www.manhattanprep.com/gmat/app/ Magoosh has a good free flashcard app here, that covers foundations:
https://gmat.magoosh.com/flashcards/math I've also created flashcards here on Brainscape, which is an excellent adaptive flashcard software (screenshot attached), but many of them may not have detailed enough explanations for where you're currently at:
https://www.brainscape.com/p/1AYKP-LH-6OXPMBest wishes with your prep and exam!
Attachments

Brainscape (confidence-based apaptive flashcard app) -- screenshot of progress overview screen.png [ 57.64 KiB | Viewed 1830 times ]

MGMAT web app -- screenshot of 626 foundations problems and confidence tracking.png [ 102.29 KiB | Viewed 1868 times ]