Okay, former T-Mobile tech support person here, so I have some knowledge but it’s out of date - So consider this ‘general’ info.
@Retzbu_Tox has the best take on the information so far, and in general the majority of the 5g rollout has been the low band, especially for T-Mobile, while a few carriers for deep urban areas are going with the super high (often referred to as mm wave) frequencies. In real world situations the low frequency 5g is 15-20% faster than the ‘average’ 4g/LTE speeds for downloads, less for uploads, and it’s still unclear if that will remain the same as there is more load on the networks, but appears to be pretty steady.
The high frequency stuff is amazingly fast, as in up to several times 4g/lte speeds . . . but in the words of my coworker, a freaking tree in the way can cut your speeds due to the incredibly bad penetration. Do not expect good performance from these techs inside a building for example. And it’s build around a mini-tower pretty much on a block by block basis.
For most casual users, you can see low band 5G as “4G +”, an improvement, but not a huge one, with minimal loss of performance inside or if more distant from a tower. Most major carrier plans already include 5G service for calls and on-phone data (up to the limits of your plan) but generally have much more stringent limitations on 5G speeds for tethered/mobile hotspot devices.
As an example, the current Magenta plan from T-Mobile only allows up to 5GB of high speed data on mobile hotspot without a plan upgrade.
I generally upgrade my phones every 3-4 years, still rocking my Pixel 3a, and would have been buying a 5a if it had been available through the carrier, rather than just Google directly (COVID has hurt my finances to the point where I can’t afford to take a minimal trade it, need those carrier incentives to make it work).
If you’re planning on buying a new phone -anyway-, yeah, get a 5G phone for ‘looking ahead’, but I wouldn’t buy a new phone to just get 5G unless there’s a stupidly amazing trade-in option. Recently (and still as of yesterday) T-Mobile post-paid customers could trade in ANY working, undamaged smartphone for a pretty basic One Plus 5g smartphone (approx $220 value), on up to 4 lines. You did have to pay taxes up front, and it’s a 2 year financing with 2 years of rebates, so it’s a retention method, but great if you have (like me) a box of old, undamaged, but working cells phones. I got rid of several old Galaxy Core Primes which were cheapos back in 2014 when they were new.