and use that computer as a "cold" wallet, with all wifi and wireless settings turned off,
You were good up until this point...
and only physically plug the laptop in when going online (20 minutes here and there) and unplugging it...
And then ruined it here...
By connecting it to any network for any period of time, this should no longer be considered a "cold" wallet.
could that be considered safe enough to be a secure place to store BTC or other crypto?
Most likely, assuming other precautions are taken... it will be "OK" from a security point of view... but by connecting this laptop to the network, you are effectively removing ALL of the safety barriers that a true cold wallet provides.
I'm not talking about connecting to wifi now and then, I'm talking about ONLY plugging in the ethernet cable when going into the wallet and unplugging it when I'm finished. If I have the spare laptop (can't sell it, it has sentimental value but I have a newer, faster one I use daily) would the experienced on here say that's a secure way to store coins?
Given that it would take a piece of malware mere milliseconds to send your wallet.dat and keylogged password to a thief, it makes no difference how long it is connected for... if you want to properly implement the cold wallet setup, the laptop should NEVER be connected to any network after it has been setup.
You should run the "online" watching-only wallet (with just public keys) on your daily computer... then the process would be:
- create unsigned transactions on online wallet (it has updated wallet balance/transactions)
- transfer unsigned transaction to offline wallet using a USB stick
- sign transaction on offline computer with private keys
- transfer signed transaction back to online wallet using USB stick
- broadcast signed transaction
At no point, should your offline laptop be connected to a network if you want to consider it a proper cold wallet.