Now I see that your main point is for ET
I would say that even warlock c2 is very good now even better than elem3 imo, just hope warlock c3 can be better.
In your situation, i would suggest you try my build, pyro2thaum3warlock2
It also a good ET build in post combat change which nobody tried it but im using it farming solmiki now.
No dps? wait, thaum3 basically made everyone into semi-dpser or make dps into godly-dpser which would make ET access less gear dependent and more fun for everybody, means more different builds/guildies can join.
The rest i can say sleep, firepillar, enchant fire, mastema, invocation, pole of agony, reversi, shrink monster lv12, all swell buffs are extremely good skill in ET and you need learn how to use them.
What you need in ET? A good ET player will not discriminate other build even ready enuf of red pots for self sustain., most builds can do it but some classes rely on falconer or cryochrono or miko made the slot more limited. But certainly 2-3 players will be core, while 1-2 players could be flexible build.
Main sustainable DPSer like dopple with 6k-8k atk/matk with 80% attribute or 2 non so sustain DPSer like SR/elememe or just 1 with cryo/chrono
^ Most team fail to get this but thaum3 build help this a lot
Good Def <- Swell Left Arm with shield can help
Good CC <- Sleep + Firepillar can help
Multihits <- Enchant Fire can help
Good Healer and Resurrect
The rest are about strategy and positioning
Thaum3 also popular in jtos since they hv no leticia cube or any RMT or afk farmer or speed leveling. Some of them did 40F with purple practo no transcendant by utilizing thaum3