You're spot on with CPU usage. However: how would you design a RasPi-efficient, fault-tolerant, decentralised ledger with strict ordering and a transparency log?
Consider CAP. Existing banking systems choose partition tolerance (everyone does their own thing all the time basically), and eventual consistency via peering - which is why all settlements are delayed (in favour of fraud detection / mitigation), but you get huge transaction throughput, very high availability, and power efficiency. (Any existing inefficiencies can and should be optimised away, I guess we can blame complacency.)
The system works based on distributed (each bank) but centralised (customer->bank) authority, held up by regulations, capital, and identity verification.
Online authority works in practice - we collectively trust all the Googles, Apples, etc run our digital lives. Cryptocurrency enthusiasts trust the authors and contributors of the software, CPU/OS vendors, so it's not like we're anywhere near an absolute zero of authority.
Online identity verification objectively sucks, so that is out the window. I guess this could work by individual users delegating to a "host" node (which is what is already happening with managed wallets), and host nodes peering with each other based on mutual trust. Kinda like Mastodon, email, or even autonomous systems - the backbone of the Internet itself.
Just a brain dump.