My most important question: Which is better, staking or investing?
I don't know what other users must have said since I've not gone through the comments, but if i must be frank with you, I did tell you that your question is unnecessary, this is because both investing and staking go hand in hand, that is, they are like the same thing, though you can invest without staking, but you cant stake without investing.
Think of it this way, for you to stake bitcoin, you have to first of all invest in bitcoin, that is buy some bitcoin , for you to stake eth, you have to first of all invest in eth, if you wanna stake ADA, or any other type of altcoin, you have to first and foremost invest in that altcoin, you cant stake without first of all investing, so staking is like an added advantage to those who invest, since by staking, you get to earn more of that coin you invested in, or any other coin used to reward the stakers.
And have it in mind that doing staking on centralized exchanges is very risky, and not just centralized exchanged but any centralized platform that offer staking is very risky , you could loss your money if anything goes wrong with that exchange or platform , better invest and hold on your non custodial wallet if there is no decentralized staking option available for the coin or token, than stake on a centralized exchange/platform.