I’ll be driving from tegel to dahlewitz. What’s the speed limit on those stretches and is there a ‘national speed limit’ sign of sorts?

Just drive at the speed most of the other cars do, then you’ll be fine.

speed limit signs look like any normal UK ones
unrestricted signs are black stripe on a white circle a bit like UK NSL
I’d imagine that section is probably limited to 130, 120 or 100, or possibly even 80 at least in sections, but I’ve not been there for years, and the last time I did, it was from Dresden.