OK they have an agenda to make it into a reskinned version of a New Hope in the past, fair enough, it is their artistic licence to work in a setting we love.
Lack of tech progress seems fair to me, there is a lot of turmoil in the SW setting to make a huge story universe. Any group that gets technologically advanced will become the target of some other faction wanting their edge.
It is shown that even in the Republic corporations for example they will run front operations at a loss while funding research programs. Something always messes it up, some other faction wanting their knowledge or to remove their advantage.
Once something is built and running for 100''s of years, 1000''s of years even the pool of knowledge dries up. No one needs to learn about it or be an expert in the field.
The galactic history of the Republic from the beginning of the old republic to the time of the new republic is only really a resurgence of mixed levels of technology after the peak of the Infinite Empire that had greater technology for a time. That even the Imperial designs of a New Hope are just recycled Sith mentality seems like great plotting in my opinion.
The Sith and Republic are locked in a tide of day and night, where Civilization and technology are built up and destroyed again in the conflict on a galactic scale.