Had almost the same grind developing on my SS and the Redline fluid swap actually bought me about eight months before the grind came back worse and I finally had to bite the bullet on a proper synchro replacement. Honestly not a bad outcome for the cost of a fluid change and it gave me time to find the right shop without any urgency pressure. The double clutching you mentioned to get into gear cleanly is a sign the synchro is genuinely struggling rather than just being finicky so I wouldn’t wait too long after trying the fluid before making a decision on the rebuild. These things tend to get worse faster once they pass a certain point of wear.