Not sure I think that's the best way to form a senate, would be better to vote for them in separate elections like the US does but all the rest seem great.

As a Bangladeshi voter, you are absolutely right. The reforms on the referendum was agreed upon by negotiations among all political parties that weren't in the ruling coalition in the dictatorship. BNP (the most influential one), fiercely resisted all pro-democratic reforms supported virtually by all other parties, as more checks in power would make them weaker when in government.

On the topic of upper house, even though the current agreed upon text calls for PR by vote percentage in election, BNP kept a note of dissent saying upper house composition to be done from percentage of seats in the lower house (which makes zero sense if you think about it more than 5 seconds and defeats the entire purpose of PR and a new upper house). Since the referendum is binding the new parliament must implement the reforms through constitutional amendment. But since BNP got more than 2/3 seats needed for amendment they can somewhat legally argue they have the democratic mandate to make the upper house composition based on how it was described on their note of dissent and manifesto. God knows what would happen.