I've seen over one dozen websites believing that BTC will skyrocket in the upcoming years.
The platform I use is Coinbase.
I'm thinking of just buying BTC-USD and just leaving it be.
Any thoughts on this strategy?
Good decision you take . Bitcoin is the best way to make some good profit with a safe and secure investment. But this the best decentralized eco system in Crypto industries. But one think always remind that the investment will safe and secure till it's on decentralized system but it will not be guaranteed when you use a centralized exchanger for holding it specially for long term. So your investment choice and strategy all are good but kindly change the platform from centralized to decentralized system for holding it

With what I've experienced, that is the kind of losses I've experienced from holding coins, not just bitcoin alone, on centralized exchanges, if it was possible to buy bitcoin from a decentralized exchange, I would have also recommended that as well,..
But then operating a decentralized exchange is kind difficult for most crypto newbies, it is very easy for most to get around the complexity of centralized exchange, than decentralized exchanges, so best is as you have advised, buy the coins from a centralized exchange and immediately withdraw them to your non-custodial or self custodian wallet, hardwallet is always the best choice by the way.