However, my receiver can't do audio in via HDMI, so that's kinda moot, I think.
I will likely try the J-tech device recommended above, which apparently grabs the audio data from the Roku, bypassing the tv, so encoding should not be an issue then.
Most streaming services (e.g. NetFlix/Prime/Hulu/Disney+) only use DD+ for 5.1 support.
Some offer both DD+ and DD, depending on selected content/detected support (HBOMax/Apple TV) or allow you to select it (Paramount+)
A few (e.g. The Roku Channel) only use DD for 5.1 support.
The SS+ (381x) does not have a Dolby decoder - it only does Dolby passthrough. It cannot transcode/convert DD+ to DD.
Because you are using the AVR's Optical in, something has to convert/transcode the DD+ (e.g. from NetFlix/Prime/Disney+/Hulu/etc) to DD for Optical (for your AVR) in order to maintain 5.1 surround - currently that something is apparently your TV (unless it doesnt and you havent really noticed/cared).
As long as the HDMI->Optical converter device has a built-in Dolby decoder that can convert/transcode DD+ from HDMI to DD for Optical, then it wont be a problem. If it doesnt have a Dolby decoder then you'll be limited to 2.0 for most services - a $15 device wont have a Dolby decoder.
If you dont care about 5.1, or are fine with limited DD5.1 from HBO Max/Apple TV/The Roku Channel apps and otherwise upmixing the 2.0 from everything else to 5.1 using DPLII/etc in your AVR, then this will work fine.