Xfinity can be deprioritized based on usage.

I've found Verizon is still fine, though maybe since 5g has taken over, there may be a few more dead areas due to signal propagation or something. I don't know if they have different frequency bands than they used to.

Just did a speed test now and got 170mbps on Verizon (4/5 bars of 5G). If something is slow where I am sitting right now, it's not a network issue.

The girlfriend has t-mobile - they are drastically improved compared to where they were 15 years ago, but there are still more gaps in rural areas. In urban areas, one carrier may be better than another in one spot and may well be reversed in another.