BUT... I have to do what works while I'm trying to improve, otherwise it means never playing things I can play, just because its not technically correct.
Basically I think that anyone should go with whatever suits best, but unfortunately a specific technique which may prove useful and easy in the beginning, could be limiting later on with higher speeds or when playing songs that need certain techniques (like string skipping 2 or more strings) that are rather hard to play then.
My personal problem with forearm picking is the stiffness of your wrist and the bad control you have over it (because of the long way between your elbow and the pick) in comparison when you pick with your wrist only, so it was always quite ****ty to use when picking slow or even when you want to play staccato by muting with your pick (by laying it on the string before the next note is picked) which I fathom to be quite impossible (or at least very hard) with forearm picking. Same goes with Artificial Harmonics or other fancy stuff. It is limiting and therefore I wouldn't recommend it.
Another thing I'd like to ask you about. I too do many exercises that are very difficult and , as you said, exhausting. But I am having a major difficulty NOT applying some tension during my exercises. You said that if there's too much tension just slow down.
I should have made that more clear. I think it is important that you actually do stress your muscles, so they have the chance to adapt and therefore it is good if you push them a bit when practicing, but when it comes to playing (for the sake of playing) then you should be able to play what you like to play without any tension whatsoever, which usually means to play the fastest you can without tension, because you want to use, and not train your muscles in this situation.
For me, playing slow at first for a while helps, to a point, but sooner or later if I don't force myself to play, especially the medium tempos (1/4 notes around 112 to 126) then I won't be able to do it. I simply can not play many tempos until I force myself to do it for a while.
I ended up practicing downstrokes only and upstrokes only on the lower E String with a metronome like a maniac. It offers the most resistance and therefore trains the most. In the beginning I really struggled to play upstrokes only 1/4 Notes even on 80 bpm (because of the pain in my forearm and that awkward motion), but afterall it got better and better and now (2 months later) I am able to play 1/8 notes at 160 bpm with upstrokes only without any pain or tension in my picking hand or forearm.
It's really just the matter to stress your muscles to do that kind of stuff and build up strength as well as endurance; at least that is my experience.
Start slow and with big motions, so really get that pick as far away from the string as possible between every attack.
If you have un(der)developed muscles you will feel pain and/or burning in your forearm really fast. If not, make it faster until you get.
It may seem dull and boring, but it helped me to get make bigger progess in some months than in years before, just because I never paid attention to the muscles I needed to upstroke, which were just extremely poor and weak in my case.
For the most part many exercises like you mentioned work for me. The frustration is that I still have to warm up with these exercises all over again every day when i start to practice. But I guess I haven't ever found anyone that said they no longer have to put in the hours.
Warming up is basically necessary before you play, so it's kinda normal that you're a bit stiff at first, although you should really be able to play quarter notes at 120 bpm even when ur hand is dead cold.
I still practice scales (with and without string skipping) with downstrokes only and upstrokes only every time I grab my guitar, just because it does so much for your hand coordination to just know by intuition where your strings are, no matter from which direction you're coming with your pick or if you have to do a down- or upstroke as well as having the strength and endurance to pick with enough force and enough feel.
At anytime try to have a loose wrist like if you are shaking off something from your hand (or doing some other stuff involving wrist-movement...
). Try to get this feeling in your picking hand when playing (You can also do that in your daily life without guitar, if you can stand people getting irritated by you.
). Every tension and pain you can get, should be in your forearm only. Your fingers should be loose too. I've seen people that cramped their fingers and/or held the pick too tight, so they eventually got pain in their picking hand.
Your wrist movement comes from muscles in your forearm and only there you should feel your muscles working (either through exhaustion, burning or even pain).
It's really hard to describe such things in text, but I was able to ignore such simple stuff for almost 8 years and got me stuck in my playing. The sad truth is, that it just not falls into place someday when you keep practicising 'something'. You can practice wrong stuff forever and still don't make any (or just tiny) progress and get stuck without even knowing why.
Guitar teachers should really talk about these things, but it seems (never had one) they often don't and just tell your to "keep practicising". Probably because they randomly learned it "right" and never thought about the problems other people are struggling with that didn't have such luck.
Sorry for this wall of text and sorry for any grammar or spelling errors. I'm just a kraut.