Thanks to @LN I found this thread - it didn't come up when I searched or during my composition... So apologies for duplication:
There's definitely a lot of overlap here - clearly more thought put in by @tomsmyth 
but I still think that the 'add new group' button might be a useful replacement to 'got to end' (no space for additional button). That is kind of covered in the discussion but is part of an additional jump screen. I guess I like the simplicity of one layout, rather than different jump screens depending on where you are in the form...
One thought that struck me is the use of a 'fly-out' side bar to navigate between groups, that I've seen in other apps but for different purposes (e.g. Qfield) - that might get round the lack of space on most phone screens, but may not be viable within the current functionality of ODK Collect?
I agree with @yanokwa that an inline dialogue would be helpful - I am currently implementing a note as the last 'question' so that my users can find the place to navigate their way to a new group when using the jump screen... But the direct 'add new' would give the addition navigational option... Does that give a 'best of both' approach?
A note of caution, prompted by my duplication:
It gets more difficult to keep up if discussions leap from platform to platform... This isn't my day job, so I have limited time to keep track of stuff - and I'm easily confused. Would appreciate any proposals or updates coming back to the forum, but accept that there could be nitty-gritty discussions at the 'development' level. No offence intended.
Thanks.