Ah, your
Blue Iris version is quite old. That is why you don't see a Group Settings option in the context menu in UI3.
Maybe you are on a Blue Iris version that does not handle switching between sub and main streams properly. What is your support plan's expiration date and your current Blue Iris version? Perhaps you are eligible to run something newer.
Or you may have your sub stream and main stream frame rates or iframe intervals so misaligned that Blue Iris is unable to get them synchronized. See this guide on setting up sub streams:
Sub Stream Guide
Can anyone see anything wrong with anything here please?
Yes, you've clearly tried a lot of things that didn't solve your problem, but you didn't set those things back to defaults so they may cause more problems for you later.
Just from what is obvious in your screenshots:
1. In the Blue Iris web server > advanced > Encoder profiles. This is all covered in
UI3's help file, but set the quality back to 50 and keep the "Limit bitrate" box checked. This doesn't even affect UI3 normally because all of UI3's default streaming profiles override the quality and bit rate limit with sane values. But the official Blue Iris mobile apps do not, so they would be sent absurdly high bit rate video streams the way you have it set in the screenshot, and they would perform absolutely horribly. This is a very common configuration error actually, there should be a warning built into Blue Iris about it IMHO.
Also be aware that Preset "veryfast" can add some video delay, so I recommend "superfast".
2. In UI3 Settings, the "Maximum H.264 Kbps" setting is not capable of increasing your streaming quality in any way. All it can do is impose a stricter limit on bit rate than you have set anywhere else. It is meant to be a convenience option, like if you have an internet connection with only 1 Mbps of speed available, you can set 800 or something in that box to help ensure the video streams don't lag.
3. For Jpeg Supersampling, a value of 2 will greatly reduce the efficiency of any jpeg images that UI3 loads because you are literally telling UI3 to load those images at a higher resolution than they will be displayed at. I recommend going back to 1.0 with it. This setting mainly exists for extremely rare cases where the machine you run UI3 on does not handle DPI scaling correctly.