I believe that by cooking the dry strawberry powder in your cookie mixture, you are inadvertently rehydrating the strawberry substance with the small amount of liquid available within your mixture, primarily from the butter. This isn't necessarily an issue, but like you noticed it does mean that you have less liquid for the rest of the cookie to make use of, resulting in relative dryness, denseness, and graininess.
Rather than decreasing another ingredient, I believe increasing the butter by about 65g (a similar ratio as what you have between flour:butter) may help with your list of issues.
This does give you a significantly high butter:flour ratio, which may cause your cookies to soften too much, but I believe that the extra, slightly absorbent material added by the strawberry powder ought to bring things a bit more in-line with your expectations.
If you're feeling a bit more cautious, you could try adding only 38g of additional butter, such that in the absolute worst-case scenario you only have a 1:1 flour:butter ratio for your cookies, which is still within relatively normal expectations, albeit a much thinner cookie, but will hopefully normalize out with the strawberry powder.
Best of luck with the cookies, and I hope you let us know how it goes! I love strawberry. :)