Playing on the galaxy site and notice my PR is affected cause I double one roll to late and recognize later Why should the rating be any different especially if the situation percentage of winning still remained at 75% on my next turn?

The correctness and result of a play are not the same thing. It sounds like waiting a roll didn't actually end up hurting you, but you still cost yourself EV by doing so. You could've had a strong sequence that blew past your market, for example.

Okay I get that if a had a strong sequence I miss the mark or if I had weak sequence I also miss the mark. But if the sequence stays even and how determine if a Gammon is possible if I cube really early ? Am I not also missing value of gammoning if I cube early?

Early in a game, it's harder to assess gammon chances, as the game can evolve in so many different styles.

I would therefore say that gammon chances typically cancel each other on each side in the early game, unless one side has already a big advantage (which should clearly be observable).

When is it “too good” to gammon?
I read 30% gammon chance is threshold is when player should “slow play their hand” and not double

That is incorrect. In money play (for simplicity since gammon value varies with score in match play), it actually depends on how likely you are to lose. It is essentially a risk/reward situation. You have a safe option of cashing the game now and winning single or the risky option of playing on and potentially winning double. Single wins when playing on are a wash, so the risk is losing, the reward is winning gammon.

On a 1 cube, playing on risks 2 points (-1 for a loss vs.+1 for cashing). The reward is 1 point (2 points for gammon vs 1 point for cashing). Since you are risking 2 points to gain 1, you must be at least twice as likely to win gammon as you are to lose. There is no set gammon probability that makes you too good. You can be too good if you win 3% gammons if you only have a 1% chance of losing. If you have 30% gammon chances but 20% losing chances, you are not too good - you should cash.