The trust problem is easy to solve, with an open society: as long as payments got processed with open APIs and the government takes care of the frauds there is no trust problem. I do not need to trust a third party with eCash, I only need to trust my State protections.
The idea is already tempted, see not only the historic eCash, witch are the modern GNU Taler chosen (it seems) by the EU for the digital Euro https://www.ngi.eu/ngi-projects/ngi-taler/ and https://social.network.europa.eu/@EC_NGI/111499172838284606 but also https://openfisca.org and https://github.com/CatalaLang/catala or few others alike.
That's still embrional but in FLOSS terms we have already more than enough, we just miss the law enforcing it and the schools teaching it to the masses.