Can you elaborate. Say I buy parts myself and install a fully FOSS OS on my machine. Let's say I want to access my bank, and they demand attestation. You propose I'd buy an off-the-shelf, universal attestation module of my chosing (free market). But how would that work from an implementation standpoint? How would the module help put e.g. my bank at ease?

Those actually exist. Yubikeys, Nitrokeys (complete FOSS FW) or bank-approved code generators (For Germany these exist: https://www.reiner-sct.com/tan-generatoren/) are basically that. They provide independent assessment. So regardless of the OS or the browser both parties can make secure transactions.