Have a look at this Brompton upgrades

The video covers the details.

This shows results and the fold Brompton mods finally complete

Note I had the shorter S stem, so the cables may need to be lengthened further. I used Brompton’s own H cables, but you can make up your own.

I don’t have your problem, but, though in daily use, always found the Brompton a tiring ride as it always felt cramped over any more than 10km. Much better now, but memory says still not as good as the Brompton copy I have in Thailand, which allows much more freedom of adjustment to the saddle and handlebars. However, the fold isn’t as good as the Brompton.