I don't think there is a minimum amount to use a hardware wallet. At the end of the day, inconvenience such as switching networks is a cheap price to pay for security. Just remember that a lot of people lose a ton of money because of silly mistakes like not checking whether they received a phishing e-mail or not, storing their password on cloud storage, etc. Hardware wallet is a good choice as an additional layer of protection for your crypto assets like ETH, it is not a rich man's exclusive tool. I'd suggest people use them even if they feel it is a bit excessive because doing so in this context is always better than doing less imo.