Highly-accreting quasars show fairly distinctive properties in their optical UV and X spectra, and are easy to recognize because of their specific location in the quasar main sequence: they are the strongest optical FeII emitters. Yet, the physical origin of their difference from the rest of the quasar populations is poorly understood. We analyzed the chemical composition of the broad line emitting gas in UV samples of highly accreting quasars at intermediate redshift, deriving very high metallicity values, above 10 times solar. At variance with the classical radio-loud (jetted) sources, we also found evidence of a thermal origin for the powerful radio emission in several highly-accreting quasars. Since the quasars in our samples are essentially unobscured or almost so, radiation and mechanical forces from their outflows must have already swept away the cocoon of gas and dust surrounding the accreting black hole in the early stages of the quasar evolution, at least within a cone coaxial within the accretion disk axis. Unobscured highly-accreting quasars at intermediate and high redshift might therefore be in a particular evolutionary stage still involving a contribution of nuclear and circum-nuclear star formation in their multifrequency properties.