I wonder if you can have a full Mina node on any device including low-end phones, keep full network state and do payments in same time? It sounds too good to be true...
Highly unlikely. SNARKs have parameters that determine the probability of validating an incorrect proof and as the volume of transactions grows the probability that a node will accept an incorrect proof does as well. So as the network and transaction volumes grows the size of the proofs will also need to increase to remain within the tolerance level required to avoid validating incorrect proofs.