As mentioned, any non-Stick supports IR.
As to 5.1 audio, to my understanding they all do, with certain limitations. The Ultra LT (4662 or 4801) is a Walmart-specific version, and to my knowledge the main differences from the "regular" Ultra was the removal of the USB port, and the 4801 also lost the DS card slot (which has been removed from all the latest models), support for AC4 audio and the Dolby Digital encoder. But either version of the Ultra LT should be able to stream 5.1 audio from any service that offers it. Just remember that some providers don't stream 5.1 audio by default, and you have to manually change the audio track. I know Paramount+ was like that before, but I no longer subscribe so can't say if that's changed.
Since I don't have either of the Ultra LT models, I can't speak for certain if the menu options for audio differ from my 4800. But I have a 4640, 4670 and 4800 and all are capable of 5.1 audio output. I only have two of them connected to an AVR, so getting more than two channels is a moot point for the rest of my Roku players.
The certain limitations I mentioned deal with media played from a USB drive or streamed from a DLNA server on your network. In those cases, the container used for the media is one factor, and the audio codecs the media uses is the other. Dolby Digital (AC-3) 5.1 audio is available from any supported container, as is DD+ (EAC-3). For DTS, it is only supported in the MKV or TS container, and no lossless audio codecs are supported in any container. AAC is converted within the Roku to two channel PCM. A handful of Roku players have DD encoders, and are capable of converting 5.1 AAC to Dolby Digital. I don't believe multi-channel PCM will passthrough the Roku to an AVR.
