mocacinno has already given a good explanation of the numbers involved, but here's a post I made on this topic a while back:
The whole security system for bitcoin is not that it is impossible (which would be good) but that it is vvvveeerrryyy unlikely.
It is impossible to have a security system which is impossible to hack, and as far as security systems go, bitcoin's is pretty darn good.
Given that most 2FA codes are 6 digits long, there is a 1 in 10
6 chance of someone guessing your 2FA code.
Assuming an average house lock as 8 tumblers, and each tumbler can adopt one of 10 positions, then there is a 1 in 10
8 chance that someone will be able to guess your exact house key shape and unlock your door.
Given a standard credit card has a 15 or 16 digit number on it, there is at most a 1 in 10
16 chance that someone will be able to guess your credit card number.
If you use a password manager to generate a long and totally random 16 character password, drawing from the full ASCII 95 character set of upper and lowercase letters, numbers, and symbols, (e.g. CY\u4"=t{rV%;N9S), there is a 1 in 4.4*10
31 chance of someone guessing it.
The chance of someone guessing your private key is 1 in 1.158*10
77.
The chance of someone correctly guessing your password, your 2FA code, your credit card number, and the key to your house simultaneously is 4.4*10
61, which is still around 2 thousand trillion times more likely than them guessing your private key.
The TL;DR is that if you are worried about someone guessing your private key, then you should be paralyzed in fear about someone guessing all your account passwords, your credit card number, your social security number, the specific radio frequency to steal your car, or pretty much anything else you keep protected. All are magnitudes more likely than someone guessing your private key.
You are far more likely to have your coins stolen from your poor security practices.
I store my encrypted seed and encrypted private key (BIP38) on paper, on a drive and on google drive (3 copies)
Storing anything in the cloud is a risk. I would stop doing this immediately, and consider those wallets compromised and move the coins out of them. Storing on a drive is only safe if that drive remains encrypted at rest and is never part of a computer which will ever go online again.
I always put flight mode on my computer and use private browsing on firefox before using bitaddress.org or ian coleman wallet or coinb.in.
Private browsing achieves literally nothing in this set up. Putting an online computer on flight mode also changes very little, as any half decent malware which requires an internet connection to transmit data will simply wait until you go online again.
And I always close firefox, delete unemcrypted files, empty the trash before removing plane mode on computer.
Do you overwrite the sectors of your hard disk which held the unencrypted data with junk data? If not, then you haven't actually deleted anything.