Is it possible to have the following features on the switch:
Link aggregation
DHCP snooping
Dynamic ARP inspection
IP source guard
By the way, the switch only have 896 KiB disk space. (It has a lot of temp space though)
If I have to install new packages for the features I need, I would lose them after I reboot the device, right?
Link aggregation needs to be done in software so far, and the SoC is certainly not able to do that at wirespeed. There might be a way to have the SoC handle it but OpenWrt does not have the code for that (the Realtek SDK does seem to, since ZyXEL offers LACP in their web UI).
I have a few ZyXEL Realtek switches, but where I do need LACP I have stuck with the ZyXEL firmware (XGS1250-12, GS1900-8HP v2).