It’s just a bank account at the end of the day, and it’s FDIC-insured, so why not?
I’m using SoFi right now (4.5%), but I’d switch to Wealthfront in a heartbeat if they had joint account support. They’re always very fast to raise the rate when the fed announces a rate hike, and they seem generally pleasant to use from what my coworkers say.
Language learning is a long, long process, and it’s important to make sure your habits are sustainable. It doesn’t really matter what’s optimal if you get demotivated and stop learning, so above all, you should do whatever keeps up your learning process. Don’t force yourself to speak the flashcards aloud if that will discourage you from the whole thing.
That, and don’t worry about optimal. There are no bad habits that can’t be unlearned (and the value you’d get out of speaking would far outweigh any effort you need to invest in the future if you want to improve your accent). Speaking would be great, but as long as you’re learning grammar and vocabulary, you’re on track.
The /r/LearnJapanese subreddit wiki is still the best place to find this kind of information, unfortunately. Maybe we start to populate our own to rely less on Reddit, but for now, I would start there (main wiki page), with a specific answer to your question being on the resources page.
For a personal answer, I’ve relied heavily on Anki (flashcard software) with a Core 2000 deck (e.g. ankiweb.net/shared/info/2141233552). There are lots of variants of the deck if you search for anki core 2000 deck, but they’re all vocabulary lists sorted by how common they are in everyday language. Super useful.