Good assassins against tank meta?

Try full ad Poppy. The only item with any AP you would buy would be Icebourne. She basically ults the tank, or support, or whom ever she can quickly that deals the least dmg. Then ideally she's free to 1 v 1 and assassinate the enemy carry. Building items like BotrK and itemizing properly with defensive boots she can stick to and take out Tanks as well. My rule of thumb with Poppy is for every time I die if I did not secure a kill I am not playing properly with her. But as long as I'm able to assassinate at least 1 target or contribute grossly to my team's favor in the fight before I die, then that's fine. The most ideal situation is being able to either continue assassinating targets, or retreat if able. And learning to manage Poppy's mana pool properly should allow you to have the tools to do just that.

Assassins do a very good job of eliminating hypercarry, and without the carry tanks simply doesn't do enough damage.

I mean, that's kind of your job dude.

Most assassins function the same before. You kill the two-three squishies on the enemy team, protect your adc or other tank shredders from those squishies, and let them kill the tanks (don't be afraid to at least help, but don't expect to 100-0 them).

It doesn't really feel like a tank meta, tbh. You got two-three targets to kill, not five. If you want to be able to duel tanks and kill them, consider playing a bruiser or an adc instead of an assassin.