Τελεῖτε ye pay is, literally, ye accomplish or fulfill carrying the sense of the fulfillment of an obligation. Φόρους tribute is from φέρω to bring something brought. Rev. makes the verb indicative, ye pay.
God's ministers (λειτουργοὶ Θεοῦ)
See on ministration, Luk 1:23, and see on ministered, Act 13:2. In Rom 13:4, διάκονος is used for minister. The word here brings out more fully the fact that the ruler, like the priest, discharges a divinely ordained service. Government is thus elevated into the sphere of religion. Hence Rev., ministers of God's service.
Attending continually
The same word as continuing steadfastly in Rom 12:12.