I tried very hard to find a seller to sell me their BTC but man, if they have it, they want to hold it! They don't want to sell, they just wanna buy.
Or maybe it's because in such a trade a fiat payment is reversible while a bitcoin payment is not and they don't want to lose money!
My Lenovo Windows laptop was purchased around 2 years ago.
10 years of files without checking where they're from.
There are 2 red flags here. First is the usage of Windows which is the worst operating system when it comes to security. And second is the fact that you have files you don't know the origin of which may contain malicious software that may not even be recognized by an AV.
2) I used a wallet program {Program 1} to generate a seed offline.
Offline can mean a lot of things in this context.
If you simply cut the laptop you described above from the internet, in that context offline does not mean secure at all.
You also need a "clean environment" for it to be secure. Which is why people usually advise using a Live Linux.
4) I closed {Program 1} but it was still installed. Connected to wi-fi for a minute, disconnected, then I deleted the wallet inside {Program 1}
Another indication that you did not use an air-gap device to generate your seed and protect it.
Could my BTC be swiped or have you managed to hold all your BTC with worse security practices than mine?
You could store your coins in a web wallet and never lose them but that doesn't mean you are safe. What you described is a weak and vulnerable setup that exposes you to various risks.
I was unaware of Bitcoin Core and Bitcoin Knots throughout this entire process so I did not use them to generate a seed. I still don't know if I need them or no.
I don't know about Knots but you can't produce a seed phrase with bitcoin core. It doesn't have that option.
Do you think it's a good idea to create a second wallet and seed with a different method, and I keep a minority of my BTC on that?
Learn more about actual Cold Storage and how to use an Air Gap device with a clean environment to generate your seed and hold the majority of your coins in that. Then you can store a small amount in any other wallet (even a hot wallet).
Also I'd advise against memorizing a mnemonic, it is not a reliable method in the long run. Think 10 years later when you have difficulty remembering a couple of words or their order and lose all the money you painstakingly accumulated for a decade!