The last time I did write a review here on GOG, I had to contact support, to demand the removal of personal information that got displayed alongside my review.
Personal information, that I am not willing to share.
And since I don't want to risk to have to do that again (and possibly even with each new review), I don't write reviews, anymore.
But...if you're keen on sharing your thoughts on a game and you don't want to be hindered by the arbitrary "max. review length of 2000 characters", simply post your "longer than 2000 characters" reviews in the particular games' subforums (best to mark it in the title as "review"), and then put the link to that review in the actual review section of the game's store page.
Maybe accompanied with a short explanantion: "since GOG doesn't allow for reviews with more than 2000 characters, I put my review in the subforum [link to review]".
That way, you can share your thoughts on any game, without being hindered by the arbitrary "max. review length of 2000 characters", AND you can even do something, that the review section does not allow at all: edit your review(s) at a later time.
Sure, that solution requires two steps (post review in the subforum AND posting a link in the review section), where one should suffice (simply put review in the review section), but I think it's actually a pretty good solution.