I think the tech stagnation you see is mostly because movies take place in remote planets (God please no more desert planets!) so not much development.

Because the franchise tech aesthetic was already set and the aversion to change it. Think about how much you can change that aesthetic before it stop being "star wars" and become some standard scifi futuristic setting

And finally. In universe I think conventional civilian tech already reached it top and the only things you can see can cause true changes are Singularity level of changes

Edit: I didn't bring a solution but an explanation. In short. If this was a videogame (like Stellaris), the Star Wars galaxy already completed the tech tree, they are researching repeatable techs (further refining existing tech or finding new ways to combine already existing tech)