It is really unfortunate that you've been having problems Richard, I feel for you. Been there in the past for sure on some really painful tool upgrades.
I did want to mention, that I've been working hard the past week or so at rewriting my Desktop apps to be all API 2.0 compliant. And I haven't seen any significant difference in project sizes. Admittedly my projects may not be as big as some others, but I do use a lot of graphics - so my projects are not tiny.
But maybe my upgrade methodology is likely different than yours. In my case I start with a new blank 2019r2 project, then just copy objects one at a time from the old into the new, "analyze" and tweak the code according to the deprecation warnings, save, ensure I can run that new bit, then repeat until I'm done.
Note: I am also saving in the source-control friendly "text" project format.
I hope you get things resolved. Good luck.