You won't have to spring for a 'premium' carrier to use some of the earliest mobile 5G service. T-Mobile has formally relaunched its prepaid Metro service, and with it comes word that the sub-brand will launch 5G service sometime in 2019. It's not certain how that will affect plans, if at all. However, it could be your ticket to next-gen wireless at a relatively low price -- and frankly, rather handy for included services like Google One and Amazon Prime.
Metro has its limitations in its current. You do get unlimited data for as little as $50 (with the risk of slowdowns after 35GB), but you're stuck with 480p video streaming and can't get more than 15GB of LTE hotspot data per month. It's possible some of those figures might change with 5G. Until then, you'd mainly want to consider Metro as a mostly no-frills service with bonuses thrown in -- which might be enough if you don't care for HD streaming or other perks.