Pakistan was never established as an Islamic state (with an official state religion). It was established as a socialist state for Muslims because Jinnah did not want Muslims to be marginalized in a post independence India, with a Hindu majority. It was only in 1973 (26 years after independence), that Pakistan's 1973 constitution, declared it an Islamic republic.Exactly how and why did Pakistan come into being? Do tell. If Israel is an ethnostate, so is Pakistan.
Now to the question of ethnicity - Israelis consider Jews to be an ethnic group (religion based definition of ethnicity) with the identity of the state being based on Judaism, with the state trying to manage demographics to be predominantly Jewish. Citizenship is also defined based on religion, where someone Jewish has an automatic right to move to Israel, but others do not.
But Pakistanis do not. Pakistanis identify as Punjabis, Pashtuns, Balochs etc and these individual ethnicities could be of any religion. Religious identity and ethnic identity are separate in the case of Pakistan. Or for that matter national identity. Pakistan also does not try to forcefully manage demographics like Israel does or create citizenship laws that are based on religion.
So Pakistan is not an ethnostate. Israel is.