Very helpful insight, it only did the 4th /5th grind a few times then quit like never happened which I thought was strange . I will try the 4th gear at 20mph tomorrow and give everyone a update !
One helpful thing to note... this is another word of wisdom from my dearly departed grandpa:
"It's a car, dummy. It don't never fix itself."
Cars can have intermittent issues, but there's a reason why you typically notice problems with a gearbox in the lower gears. They spin faster and require more force from the clutch to stop (so you can shift). In my 70 429SCJ Torino, I could shift that Muncie gearbox 4spd car without the clutch between 3rd and 4th. It sounded horrible, but you could bounce it off the synchros a couple of times and it would go into place. I learned that after blowing the clutch out and drove it 40mi home on only 3rd and 4th. This is because there transmission gears are rotating much slower as the ratio drops. So, when a transmission starts to go bad, whether it's the clutch, linkage misalignment (old cars), slave cylinder or throwout bearing, you'll almost always see problems first in the R, 1st, or 2nd gear range. You almost always blow synchros between 1st/2nd (or R-1 on most 6 speeds) as they're the most used and most abused. R is another common one because it's a very small gear and will have a very high ratio similar to 1st gear. It's very rare to have initial synchro problems in the latter gears prior to the lower gears unless the parts or assembly worksmanship are defective.
Because more force is needed to brake the transmission for shifting (clutch) in the lower gears, they're usually the first to start grinding. At a very high level, when you push in the clutch pedal, the pressure plate mashes the clutch against the flywheel, which results in the transmission's input shaft to begin slowing. If the slave cylinders, pressure plate, clutch, or throwout bearing are not in good working order, it becomes more difficult for the transmission to slow the input shaft. The input shaft has to be slowed more for R, 1, 2 than for 3, 4, 5, 6. Hence why you will generally get a bad R, 1, or 2 first. If multiple gears aren't working, especially the higher level gears, this is indicative of a part failure that is common to all. Each gear has it's own sleeve, hub and synchro. So, when 1 gear has problems but all others are fine... your problem is localized. In your case, it seems like it has problems across a broad spectrum (even if the 5th gear issue is minor and sporadic). Thus, the throwout bearing, slave cylinder or clutch is a likely culprit.
So, back to my granddad. "It's a car, dummy. It don't never fix itself." Your problem is not going to magically fix itself. You might luck out and have bad oil in the gearbox, which is where you get temperature based performance issues. But again, the 5th gear sporadic grinding thing and not 2nd or 3rd isn't what you'd see with a lubrication problem. As I mentioned before, the gears spin faster in 2nd than they do in 5th, thus a lubrication problem would almost always present itself in 2nd and sometimes in 5th.
This issue of yours is doing my head in, as it is presenting in R and in 4th-5th. Generally, you should also be having problems from 1st to 2nd as well if the clutch or one of the connected parts are bad. The fact that you have to shift slowly into R tells me that the output shaft isn't stopping during the idle gear, as it should. This almost entirely rules out a bad synchro as being the problem (as I don't believe your M6 has a synchro on reverse, as most manual transmissions use a 3-gear system to drive the output shaft in the opposite direction). Thus, it sounds more to me like a clutch or slave/throwout bearing.
If the throwout bearing is going bad, you can hear it. When you depress the clutch, it kinda sings, in a muted sort of way. Ideally, there should be no audible sound coming from the transmission at all. As the throwout bearing goes bad, it will start to make a noise. It's difficult to explain, but it starts off sort of like a quiet rushing noise (like moving a cardboard box fast across a concrete floor), but will start to make a more familiar "bearing whine" sound as it gets REALLY bad. If everything is totally quiet, it could be the clutch, the hydraulic system that engages the clutch, or the pressure plate. A bad slave cylinder usually presents itself when the car is hard to shift OUT of gear (as it is primarily there to aid in the disengaging of the clutch). I don't think this is your issue.
Sorry for the rant, it's 3am here and I can't sleep. Any factual inaccuracies or content too deep off the nerd ledge are hopefully not too bad
