'The usb can't sign on its own, so it needs to be stuck in a computer. That becomes the risk.' The USB stick uses the host computers hardware and controls that hardware. A good quality USB stick used only for this purpose should last many decades. The signing is done remotely by transferring the unsigned and signed transactions between the online and offline devices.

Agree you definitely want to back up the seed phrase as well as you should with any cold wallet. The seed phrase storage is the most important set of all and often overlooked by people depending on HWs.

Here is one way to store seed phrase cheaply and conveniently- tip if using this method is to use a pdf ebook version and store it in several locations ie usb stick and maybe on cloud/s storage. https://www.reddit.com/r/BitcoinNewZealand/comments/zyvngp/low_tech_low_cost_secure_seed_phrase_solution/