You can connect DDR4 Memory for PL also, which is available to use in the PL block design as a FIFO or big storage like BRAM and it has several GBs free spaces. Unfortunately PYNQ-Z2 board will not able to provide it, it would be better if you start to use a better ZCU104 dev board at least, it has a SODIM DDR4 slot for PL memory extension.