Dale,
I've just tested the default Roku profile on my test server, and compared it to the profile you have posted here. I loaded a Blu-Ray rip, so HD video. It took a long time to load (maybe 15 seconds), and I encountered buffering within 10 seconds of playback beginning. The buffering continues to occur after a couple of minutes at fairly regular intervals. This could be because my test server has much lower power than my main server, and can't transcode as fast. Hitting pause I can see how far the transcoding has gotten ahead of playback. FF and rewind work, but again take forever to reload. Before I switched profiles, it loaded in about a third of the time, but I still encountered buffering.
Loading a DVD rip (SD) loading time was about half the HD loading time. I was surprised to see captions, but then I realized it was a file I burned them into to video.

Haven't seen any buffering, so it looks like my test server can keep up with SD video.
I switched FFMPEG from the default version installed with Serviio 1.5.1 with a 64 bit version dated Jan 15, 2015. The SD video still took a while to load (compared to the MP4 below) but I have seen no buffering. I switched to the same HD file. Took 35 seconds to load (I timed it this time), and buffered in the first 10 seconds. Every time I pause it the transcoding is about 30 seconds ahead. However, it looks like transcoding still isn't keeping up, as it's now down to 23 seconds. It does appear that the 64 bit version of FFMPEG can keep up better. I was three minutes in before it buffered again.
I have one HD MP4 with L4.0 encoding. Therefore it plays without transcoding. It loaded quite quickly, and is playing without buffering. According to MediaInfo, it has an average bitrate of 16.5 Mbps and a max of 25. At least for the first few minutes I've played it, there's been no issue with the bitrate. How fast do you think a 2 XS can handle?
So, transcoding to HLS slows the loading time significantly. I hope Zip (or someone else) can offer some suggestions about the control you've asked about.
Dan
LG NANO85 4K TV, Samsung JU7100 4K TV, Sony BDP-S3500, Sharp 4K Roku TV, Insignia Roku TV, Roku Ultra, Premiere and Stick, Nvidia Shield, Yamaha RX-V583 AVR.
Primary server: AMD Ryzen 5 5600GT, 32 gig ram, Windows 11 Pro, 22 TB hard drive space | Test server: Intel i5-6400, 16 gig ram, Windows 10 Pro
HOWTO: Enable debug logging HOWTO: Identify media file contents