Is anyone else having buffering issues?

Just as a note:  30.9 Mbps is not 'great' for home internet.  "Good" service might indicate that you're on either N71 or LTE B12/B71 and capacity is not so good.  

Need a little more info from your “TMobile Home” or TLife app to know if its a signal, power level, noise issue.

Eg.  Mine can show 'good', and obtain 500Mbps/15Mbps.  Excellent (open windows) can give me 550-650MBps/70Mbps.

This is because I’m ~1200’ from a tower that has a range of ~¾ mile.

It may be worth looking into a Waveform antenna (https://www.waveform.com/pages/supercharge-your-t-mobile-5g-home-internet#product-buy)

90 day return

If you're in a busy area - it might be something more like deprioritization.  I'd recommend calling T-Mobile's Home Internet support - 30Mbps is considered poor.

Yes started a few weeks ago and has gotten progressively worse.  Called support what a joke!  First person spoke so fast I couldn't understand. Tech person kept telling me I was breaking up. Even had me reboot my phone, it's on t-mobile. I can't  watch a program without it stopping and starting every few minutes. Seems to affect all streaming services and multiple TVs.

Have indeed used Speedtest to check on things. Only getting 30.9 mbps downloads currently. Did have the gateway upstairs (as recommended) though none of our devices are up there. Moved the gateway downstairs. It's now within 10 ft. of the television, and says it is getting a "good" signal. But the buffering issue seems unchanged.

Have you measured your download speed at the back of the TV and also near your gateway?  Perhaps the TV is too far away and perhaps you could find a better spot for the gateway.  (Use the Speedtest by Ookla app on your phone, connected to your Wi-Fi.)

If you home is larger, you might want to try adding a mesh network.  You can do this on your own, or I think T-Mobile is now offering that as an upgrade to TMHI service.