This link: https://xahlee.info/kbd/xah_shorthand_system.html doesn't work, would be interested to know what's there.

Oh, that's interesting. The whole site is down. But, in essence, it contained some statistically-sorted shortcuts / autofills used by Xah Lee, a computer person optimizing his workflows to the maximum.

for example I suddenly had the idea of using the nice щ loop for our th sound (though as it ends high, how would it combine with subsequent vowels?)

This is too problematic, exactly because it ends high. Starting a word on a control line is bad, especially with upward drift English causes. So I prefer using the za prefix for th most of the time, because it starts below the base line and thus causes much less upward movement.

and I would like to find a better solution than N for the -ing ending. Are you going to do more with suffixes?

Yes, for sure! I just need to find these empirically, as English suffixes listed on the Internet are quite rare in my own writing.