The crack on the cap is okay? Everytime. I use max 6nm..,....

Above 4 pics,I use 5nm,5nm,5nm,3.7nm,is it ok?

The values are printed on the parts. In general, the way the torque values are stated indicate the intended result. In your case, max torque 6Nm indicates (your torque value) < 6Nm.
Shimano publishes most torque values as: 5Nm < (your torque value) <6.5 Nm. Your stem cap and stem bolts are usually something that don't need constant adjustment once at the recommended torque value, and "topping the torque off" can break bolts and crack carbon.

Setting up your headset is something you usually do once and only revisit if the headset loosens. My stem caps are only tightened to bottle-cage torque, or 2-3Nm, and that's just to keep the bolts from working loose.

The only bike in the world that should have the headset top tightened to a specific value is the Trek Domane w/ front Iso Speed. You NEVER should tighten a normal headset to any specific torque value. You properly adjust it...that is all. I NEVER use a torque wrench on a headset unless it is the aforementioned Domane. Manufacturers put max torque on parts like that to keep idiots from breaking them or making them so tight they won't turn. The top cap is cracked because you're over tightening it.
ETA: This thread should obviously be in 'bicycle mechanics' not GD.

This.

Also you should look into how to tighten the headset:

• Loosen bolts in picture #1
• Remove the cap, make sure that the plug is at the level of the bolts in picture #1 and tighten enough so it doesn't slide in the tube.
• Put the cap back on and tighten bolt on picture #2 enough to remove the play
• Tighten bolts picture #1 to 5-6 Nm.

Modern headsets with angular contact bearings need a significant preload applied, so tightening the top cap bolt should be done before the stem clamp bolts are tightened. 5nm could be too much. If the front wheel doesn't self-center after a turn, the preload is too large. Too little and you may hear a thwack sound when a sharp mismatch in the pavement is hit. That’s too loose, but no play will be felt. To adjust preload, the stem clamp bolts must be loosened first. It's the stem clamp that retains the setting. I've been wrenching for over 50 years now. I've never used a torque wrench on any small M5 or M6 bolt on a bike. I use a torque wrench on the big stuff like a threaded BB or cassette lock ring. I've never broken a carbon part or damaged any threaded part.

I seldom use a torque wrench on a bike. I just tighten till the thing won't move. If bearings are what I'm tightening on, till enough of the play is taken out of the bearings. When I've gone back and checked, I'm always well below the torque specs.

I'd say you're doing it backwards. No need for torque wrench on the big stuff, but definitely on the small fasteners and carbon parts.

Not if you're skilled. I've owned carbon frames and bars since they were a new thing. No problems ever. Nothing slips nothing breaks.

Nothing wrong with being 68. I keep up with the times. Two disc brake frames with sram force axs, except for the cranks. Campy 48/32 and shimano grx cranks provide more range. Always trying something new.

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How often do you ride ur bike? I want to cycling until 70 years old

Are you checking out at 70 yo?

I don't plan to retire from cycling. I might get too feeble, but at that point a recumbent trike is the plan. Hopefully I'll cycle till I check out or am put in assisted living.

Even then maybe they'll be starting to make autonomous electric trikes I can ride!

I ride a 52 mile loop with over 3000 feet of climbing in the first 22 miles, 3 times per week, weather permitting. I have other hilly routes for winter and spring. Rode 5300 miles last year. Both of my knees were replaced in 2017.

What does check out mean?

Death

A.K.A. kick the bucket, buy the farm, etc

I would say <= 6Nm rather than <6Nm. I think people overthink this too much. If is says 6Nm, just set the torque wrench to 6Nm

"Less than" is ambiguous... 1Nm is less than 6Nm right? Would that be ok?

In a word: Yes! The components bearing markings like "Max 8 Nm" are telling you that torque exceeding that maximum will damage the component. It's different from Shimano/Campy/SRAM giving a minimum and a maximum amount. If it's tight enough to hold in place without exceeding the maximum, then that's the correct torque. If you can't get it to stay while approaching max torque, add a carbon assembly paste to add enough friction.

One caveat, torque ratings I believe are typically specifying the max that the bolt and the threads on receiver combo can withstand and avoid stripping. Potentially a max torque rating on a component, if used, could potentially damage the item that the component is being bolted to. For example, a seatpost clamp max torque rating, doesn't accommodate or know what material or can be withstood by whatever seatpost you're using. A stem's handlebar clamp doesn't know what handlebar you're using. A FD clamp doesn't know what your seat tube can withstand, etc. The advice to use the least amount of torque necessary to hold is the best advice.
Obviously (hopefully) CF paste shouldn't be used on threads/bolts - eg. the OP's headset -- I can't imagine there's anywhere that you'd use CF paste in that scenario.

I would assume/hope the max torque shown would have a degree of safety factor built in; if parts will crack at 6Nm, you tell customers to stop at 4Nm, because if you say 6, there's always going to be someone who goes to 7 "just to be sure it doesn't loosen up."

Skills have nothing to do with torque. Torque is not a skill it is a specific number that can only come from a calibrated machine designed to give you torque values. I can certainly tighten a bolt and take a guess at it and maybe it is right or maybe it isn't but I certainly wouldn't ever be able to tell someone torque by using my hand. Say one day I hold my hex key tighter or looser or use a different hex key with a different length or weight or something like that, what then?

I would say use torque wrenches on just about everything that is fairly mission critical or has a recommended torque spec from the manufacturer minus of course preloading bearings as that is not a torque thing (except maybe with the Trek bikes cxwrench mentioned) Using my torque wrench doesn't take much time (and once I get my box completed and figure out the wrench I want to stick with and the bits I like it will be easier currently is in a molded case at the bottom of the box and not sure I love love love it)

Maybe if it properly tightens the bolt without it coming loose and will not cause damage. The max is there so you don't go above and cause damage but if you can get away with much lower torque go for it. That is the whole idea.

This is nonsensical. How do you know it's "properly tightened?" How do you know it won't come loose?

Are you suggesting one should experiment with lower values to see if one can get away with it? If the handlebars come loose on a fast descent, oh well, I'll tighten it a little more next time and try again. Sounds stupid. If you have a torque wrench, why not tighten to specified value and be done with it. No need to experiment.