Maybe worn out synchros? it just seems like it was from maybe 4 or 5 to 100 really quickIs it possible that you have worn out synchros? Maybe this problem developed more as you were troubleshooting these last few weeks/months and grinding in and out of gear.
I occasionally have a little trouble getting into reverse when my transmission is cold and it depends on what gear oil I'm using. Have you tried the Redline LSP cocktail at all? Which gear oil are you using currently?
Did you replace the release bearing sleeve that bolts to the transmission around the input shaft? If the grooves created by the release bearing are deeply worn it can restrict clutch disengagement near the top end of it's travel.
Surprisingly tire pressure plays a small role in grinding through gears while moving. If there is an imbalance in wheel speed due to tire wear or tire pressure it can cause a slight bind in shifting. This was something I discovered around my 40k mile mark when I started grinding into 2nd and 3rd. Even tire pressure would alleviate it to some extent. This is only affects grinding while rolling however. I presume you are experiencing grinding into 1st and reverse while standing still? Downshifting to 1st while rolling will grind unless you are traveling 15mph or less it seems like.
I've been using Redline MT-85 GL-4 for over a year (the whole time I've had the car)
I didn't replace the sleeve, but I inspected it and it had very minimal wear
my tire pressures are at 32psi
yes grinding is at a stand still only, but 1st doesn't grind, only reverse because of it not having a synchro, what I've found out after all this is that when you start the car if you put it in gear before it's idled for more than around 3 seconds it's smooth, but if you wait longer than that its difficult to get in gear and reverse grinds, its the same issue with reverse not grinding when coming out of a gear and immediately going into reverse, something to do with it sitting in neutral for longer than a couple seconds is building the resistance