Analog to 8bit r2r DAC

I want to make an bluetooth speaker project and i want to convert 10bit ADC input to 8bit DAC, is that even posible?

divide by 4?

Anything can be done. But in case this is going to be about something like an Arduino Uno/Nano acting as an audio DSP - don't go there. It's not going to give any appreciable sound quality.

I want to switch between aux 1/4 jack and bluetooth and output analog signal, and them make another stages with analog effects and amplifiers

I'd start by drawing a block diagram of your proposed system and then decide on the technology requirements for each block.

I understand every block necessary for now, but i dont know how to output A0 input to D0-7 for R2R DAC stage

PORTD=analogRead(A0)/4;

Are your R2R resistors better than 0.5% precision? If not, you won't be getting 8 bit precision from the DAC.

If you use an Arduino Nano, for example, but only for control and in combination with analog multiplexers, you will be able to choose the sound source. For more complex control I would recommend an audio processor like TDA7439 or PT2314E. As a power amplifier I would recommend TPA3116D2 - there are both modules and finished products with it.

TDA7439

PT2314E
https://datasheetspdf.com/pdf-file/806069/PrincetonTechnology/PT2314E/1
TPA3116D2