if youre good, then wait for the 4d tournaments and get at least 10 wins(if less than don't buy)and buy the bonus rewards. its worth it as long as you reach the legy king chest. it's 500 each so itll take a while, but so worth it
What do I do with my gems?

I always get like 5 wins because I always play it secretly, and my parents check a lot so yeah lol. Now, I don't play secretly that much anymore, so I might get over 10 wins as well

It's so hard to get above 10 wins when playing secretly because every time we play challenges my mom just checks what we're doing. It's like she senses it from a mile away. And then we get like 3 losses for no reason at all. And the other two losses are from nervousness of playing secretly.

I honestly don't really care
I just missed the rare book of cards because I couldn't play for a minute more

yeah
I wanted the rare book because I would've been able to max out my hog rider, then my musketeer in a bit

Try to level up the more level dependent cards in your deck. By contrast, you shouldn't spend invest your gold and book to a less level dependent card. If you have level 13 fireball, for example, you can keep it at that (most interactions don't change). I don't know what level you have for the 4 cards that you need to upgrade though, so take this with a grain of salt.
Same, I'm all about cosmetics lol