A good reading why not LGPL is
here.
There is no problem of releasing "our" code as GPL on the top of LGPL libraries (libftdi and d2xx). As per LGPL, other portions of the project are permitted have other licenses, even being commercial.
But other way around, i.e. releasing LGPL code on the top GPL libraries would have not been possible. For example if libftdi or d2xx were GPL we would have to be GPL.
To summarise, releasing as GPL is fine, it will not break the LGPL license of libftdi and d2xx.
I will need to do more reading to remind myself what the difference was between GPL2 and GPL3...