Hi and welcome.
Let me start with the backlight issue.
You should reduce the current to a backlight LED, not the voltage.
If you reduce the current, the voltage will follow, but not the other way around.
To reduce the current you could use a resistor.
Lots of displays have such resistor built in, but by far not all of them.
So find out about that for your display.
In case really nothing happens, assume you need to adjust the contrast potentiometer.
You should see some contrast where the characters should appear.
You can set the contrast already when the module only gets power (and is not controlled in any way).
In that case you should see all blocks in the first line, and none in the second.
After doing that you can go and try to get some characters on your LCD.
The module has an excellent manual, telling how to build it, but also how it is connected to the LCD.
It is in German, so if someone doesn't understand that that would be a problem.
I'm assuming you understand the German language.
On top of page 4, a schematic shows this connection.
This has to be the same as in your declaration of the screen in this line:
LiquidCrystal_I2C lcd(0x20, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE);
I didn't bother to check that , but you should.