Hi!
The documented limits about FDB states that it to support up to 100TB of data https://apple.github.io/foundationdb/known-limitations.html#database-size. That is 100x times more than what WDQS needs at the moment.
"Support" is such a multi-faceted word. It can mean "it works very well with such amount of data and is faster than the alternatives" or "it is guaranteed not to break up to this number but breaks after it" or "it would work, given massive amounts of memory and super-fast hardware and very specific set of queries, but you'd really have to take an effort to make it work" and everything in between. The devil is always in the details, which this seemingly simple word "supports" is rife with.
I am offering my full-time services, it is up to you decide what will happen.
I wish you luck with the grant, though I personally think if you expect to have a production-ready service in 6 month that can replace WDQS then in my personal opinion it is a bit too optimistic. I might be completely wrong on this of course. If you just plan to load the Wikidata data set and evaluate the queries to ensure they are fast and produce proper results on the setup you propose, then it can be done. Good luck!