As the title says.

Do I get the advancements over time or am I stuck with low tech if I choose it?

Yes, you stay with the tech level you start with. Can be fun to play the lower ones, but combat typically seems to take longer because the ships are not as powerful.

This is true. Another issue with the tech level is it alters the difficulty curve. Some races have distinctively poor ships in the early eras when both compared to its own newer classes and the contemporaries of the other races. The Federation seems to have a very slight edge in the early era.

Base attacks can be a bugger in the first era... the feds have it better because of trops but on the hole yes.

Early era is pita for races that favor the missle, you can outrun them all day long, unless they get significant damage done to their engines. Levels the playing field so Photons dont suck.