Buy Monero as an investment or use Monero as a tool?

Liquid and Lightning have different trade offs to Monero, but if it works for you more power to you. There is nothing mutually exclusive about saving with Bitcoin long term, as you said, and using something else as digital cash for the short term. It's been a toss up at shorter time scales as to which performs better.

I see zero drawbacks to using Liquid and Lightning. Do you?

Come on now zero drawbacks? Everything has drawbacks. Well, let's start off with Liquid. No sender or receiver privacy (only amount privacy), virtually zero anonymity set because no one uses it, no p2p markets/merchant adoption, and custodial. It's a worse Monero on almost every dimension, besides longterm price, but if you care about that why not just hodl Bitcoin securely? UX on Lightning sucks. Inbound/outbound liquidity is unintuitive and clunky. The larger the payment the more likely it is to fail at routing. To acheive strong privacy (from both counter parties and third parties) and actually retain sovereignty you have to jump all these hoops which is why vast majority use custodians or LSPs that diminish your privacy, permission your transactions, and the former can rug you.

liquid's security model is worse than even solana

One solid channel to a large node like Kraken is enough for most cases. Or pay lightning invoice from a liquid balance via Boltz swap (we offer a choice of backends). Liquid's confidential amounts and assets offer adequate privacy after a couple of hops, such as trading BTC/USDT at SideSwap. But I agree, receiver's privacy is more of a challenge, hence darknet's vendors choose XMR.

If you’re holding Monero you are getting robbed. No one wants to Hodl XMR. And when you’re in and out of saving in bitcoin you’re gonna leak privacy. Monero is badass in so many ways, Just none of them actually hold it as a store of value.