I use a 2020 Roku Ultra, and the DirecTV Stream quality is excellent. There were some issues when I first made the switch in March where it would buffer and crash at least once a day, but I haven't had any problems since a major app update in mid July. With that update, they added a more robust pause feature with a 60-minute pause buffer that allows full trick play functions. My experiences and screenshots were the basis of this article:
AT&T TV has pushed a major update to Roku users that makes time shifting easier. Customers can now pause, rewind, and fast-forward Live TV across channels, with AT&T TV on their Roku streaming player and Roku …
thestreamable.com
Another advantage of the Roku app over FireTV (I have a 4K FireTV stick) is the Roku app has preview thumbnails when you FF/RW recorded programing, as you can see in the images in that article.
I never use my 4K FireTV stick anymore as the DTV Stream experience is so much superior on Roku. I'm not familiar with any "best" settings on either device. I just hook it up and it works. If I have PQ issues, I toggle my TV's preset image settings.