That page is for an older version of Dorico – here’s the equivalent page for Dorico 5.1.
If that still doesn’t help, and reapplying the playback template you’re using doesn’t help either, perhaps share your project or further information: what sounds are you using, for example?
I’ve removed most of the music, and yes, I still have the same playback issue. smh for using the 3.5 instructions earlier - I’m sure it’s something I’m doing or not doing. Thanks, all!