How to unlock the secrets of endgame builds?

Playing melee end-game is not a problem at all. Only problem is that you have to choose main skill from which you can gain most and build around it.

Your main problem is that your damage with either Sweep or Ice Crash is doing "rainbow" damage (multiple different elements) and that's not really good in general, unless you can exploit it somehow.
You deal cca. 50% of fire and 50% phys damage with Sweep, with that, your effective damage against any mob with fire resist mod will be a lot lower.
Same for Ice Crash, where you have even cold damage and that makes it less effective against anything with either fire or cold resist mod.

The solution to dealing elemental damage to resistant mobs is using X Penetration gem. The issue is, that in either case of Sweep/Ice Crash you would have peneration on only small portion of final damage, which is not effective.

If you want to use Weapon Elemental Damage (WED) support gem, well, deal elemental damage in general, i would suggest you to use Ice Crash instead of Sweep as Sweep deals pure physical damage as base damage, while Ice Crash deals 50% cold damage right from bat, thus Ice Crash will benefit a lot more from WED gem.

You also want to use Added Fire Damage support gem, which is huge boost number wise, but it will make Ice Crash to go "rainbow".
But there is easy solution to this. Use Pyre unique ring to convert all cold damage to fire damage.

Pyre will make your Ice Crash's final damage deal only fire damage with bit of physical. (note: I achieved 87% fire to 13% phys damage ratio with my previous league Ice Crash + Added Fire Damage + Hatred + Herald of Ash + Pyre combo).
Since, now you have larger portion of final damage doing fire damage, you can use Fire Penetration gem to deal with fire resistant mobs and also boost damage against non-fire resistant mobs.

Pyre with Herald of Ash also deals with problem of smaller AoE radius of Ice Crash once you choose to use Concentrated Effect (which is huge boost for Ice Crash being it pure AoE skill), due to anything you kill within Ice Crash radius spreading overkill damage as burning to surrounding mobs. Also taking those 3 "increased AoE radius" nodes in Templar tree compensates a bit for loss of radius.

Regarding your defense. 20k+ armour is more than enough, you don't need Determination, use Hatred for extra damage instead. I played Talisman league with 14-15k armour + 6 endurance charges and was doing fine (without +armor flasks). And i'm talking about stuff like killing twinned T15 Daresso. (With new Ascendancy stuff it will be breeze now :p)

Regarding clear speed in end-game as melee. It's about your preference and play style. If you prefer to rush through maps with only goal to kill everything asap, then ranged with 100k+ dps will be your target. But when you are also picking stuff from ground, it doesn't matter if you kill whole screen at once and then visit all dead packs to pick stuff, or you kill each pack with one hit at its location and pick stuff right at spot. Not much difference in time spent here. Just Leap Slam to pack, hit, Leap Slam to next pack, hit, etc. boring same way as shotting whole screen at once.