---->I hope you are right. Otherwise, tendonitis will tell.
Let's hope not

---->Let's define the term tension in this context: There is normal muscular tension (the one that holds your head and your torso up all day long) and there is abnormal muscular tension, the one that feels crampy and the one we are concerned with here. Normal tension does not lead to fatigue. By the way, the wrist cannot have tension, because it is a joint.
To be exact -- there must be some simultaneous contraction of antagonistic muscles that control the wrist such that a certain degree of fixation results. Otherwise any force applied to the keys by the fingers would push back against your wrist and minimize any sort of efficiency.
Granted, I agree that excessive muscular tension is bad. But it seems to me people come up with all these silly ideas about relaxation to eliminate these tensions when doing a few excercises will help them more.
---->Normal tension goes away immediately after ending the activity. Abnormal tension is more persistent (i.e. cramp-like, static) and can still be felt afterwards, and even before you started the activity. Just check yourself out. However, detecting tension is very difficult if you don't know what to look for. Try to lie down and relax completely. If you have not done this ever or haven't practized it for a long time, you'd be amazed how much tension there is still left in a body. That's bad!
Right, but when we wake up after a night's sleep, are muscles are in a much more relaxed state than other times, and at least, for me, that is the worst time to play the piano (until I am sufficiently "warmed up")
---->Regarding the term "proper technique": To me, it means - in a nutshell - getting the appropriate sound without harming the body (not the pianist's nor the listener's

This includes "no tension", as tension is harmful and, over time, leads to many ailments that pianists are plagued with. Period.
When I watch clips of the great pianists play, I can see the necessary tensions in their bodies -- they are not completely relaxed at all. Watch Horowitz play the D# minor Scriabin etude -- I doubt ANYONE could do this without muscular tension.
---->Now, that IMO is incorrect, at least what concerns piano playing. During piano playing, muscles do not need to work at full capacity. Any healthy person has more than enough strength for that. I cannot see anywhere in the repertoire where any muscle would have to work at full capacity. Not even during practicing, unless it includes pushing the piano around. If you find any of your muscles working at full capacity, your technique is flawed, and you need to stop immediately! I can see that strength training would give the illusion of improvement in this case, but it is merely masking bad technique.
You never get tired when you play?
---->Every pianist should examine an illustration of the muscles and tendons of the arm and hand. A picture gives a vivid sense of the length of the tendons and their vulnerability to various kinds of mis-use. Looking at an illustration makes clear why awkward positions, excessive force, co-contraction, and static muscular tension (the four principal causes of injury described in "Pianist's Injuries") will put extra stress on the tendons. Pianists and teachers with this knowledge will be better able to prevent or recover from injury, and better able to help their students. (Good illustrations can be found in Netter, Atlas of Human Anatomy, or in Gray's Anatomy which is available online.)
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https://www.pianomap.com/tendonitis.html)
I've read quite a bit about this.