You are not investing in bitcoin. Bitcoin is not an investment.
To OP if you're confused by this, we can confirm that @DarthCoin speaks the truth. The key is understanding the difference between investing and savings.
To save money is to put it away, in a safe place, with the goal of using it in the future. You are not giving it to anyone such as a company (shares) or government (bonds). It's not exposed to the risk of loss and it produces no returns. If you save $100 under your mattress a year later the same $100 bill will be there, it won't have turned into $110.
Investing, by comparison, is putting money to work for a period of time in some sort of project or undertaking in order to generate positive returns. For example, investing in a company and receiving their income as a "return". In involves the risk of loss as that same $100 could go to $1m or $0. The more risk, the more return (usually).
Bitcoin is not investing because it doesn't generate any yield or return. You store 1 BTC in a UTXO and 100 years later the same 1 BTC will be there. It is savings.
True savings is something most modern day people have long since forgotten the true meaning of as it's now been conflated with "investing" due to the fact that one cannot "save" in fiat anymore. Inflation will rob your savings and when you come back in 100 years there will be nothing left. Sure, the physical $100 bill might be there, but it will be worthless.
While some might say @DarthCoin is being silly in that both words essentially mean the same thing these days, it's very beneficial for people to fully understand that there is still a difference. The ability to simply save is one of the core things Bitcoin gives back to all humanity. Freeing us from the endless debasement of central bankers.
One of the top rules of computer security is: Don't role your own
Don't try and come up with some super smart, custom made, super stealthy way to "totally outsmart everyone" with. You will screw it up. You will forget your passwords. It won't work how you think it will. It will have huge security holes in it. No one else will be able to help you fix it. It will end badly.
This is particularly pertinent when it comes to full computers with full operating systems. They have huge attack surface areas, even when off line.
Instead, use the heavily battle tested industry standard of a Hardware Wallet and a well respected software wallet like Sparrow Wallet and ensure it's fully backed by your own Full Bitcoin Node.
We'd also point out that Trezor isn't that great either (although their Safe 3 is a step up). There are far better options and you can view, filter/sort and compare all hardware wallet options here. Tap the "i" button to see full specs and ratings. Our top rated ones are the BitBox02 and Foundation Passport.