by 5.30 or until 5.30/ why future tense?

Solution 1:

As a preposition of time “by” means “on or before”.

I will give you the necessary files by Sunday.

This means I can deliver them e.g. on Friday or Saturday but definitely not later than Sunday. Use “by” when you refer to a deadline.

“Until” means “up to a particular time but no sooner than that."

The shops are open only until 5:30.

They will not close sooner than 5:30.

"I think I'll wait until Thursday before I make a decision." is correct. Explanation is the same. "I am not going to make the decision sooner than on Thursday." "By" would mean on Thursday or sooner which does not make sense.

Solution 2:

I would like to point out the two confusions in your questions: until vs by, and the use of will in present tense (by now).

We use until to talk about a state that will continue up to a certain moment. We use by to say that an action or event will happen at or before a future moment.

Will can express certainty or confidence about present or future situation.

Tomorrow will be cloudy.
There's somebody at the door. -- That'll be mary.