Wow, this is really paranoid indeed
But better safe than sorry, right?
From my experience I can advise you to buy good old IBMthinkpad, maybe
X61? or X41(even older) They are relatively cheap, with descent OS, like Linux, firewall
protection and a bit of brain you will achieve security level of hardware wallet.
I have in fact both of those laptops, they are somewhat cute
One is for wallets other for surfing. Power efficient, fast (you would be surprised)
and both well supported by Linux OS. Every thing works out of the box with almost any distro! I think they may increase in value over time, kinda indestructible equipment.
X41 (10years old) boots up in 25 sec. Uses 16-24 Watts, very quiet notebook.
(battery gives 1hour of work, still not bad after 10years)