There is absolutely nothing wrong with

Since it's raining, wouldn't it be a good idea to bring an umbrella?

Look at a few other constructions to see how natural it is:

Since you're going to the store anyway, wouldn't you like to pick us up some ice cream?

 

Since our success is uncertain, wouldn't we be well advised to consider how to minimize our risks in case of a downturn?

The first clause assumes a fact, and the second balances it with a speculative counterpoint. That is grammatical, unexceptionable and, incidentally, one way to make a sentence interesting.

There is absolutely nothing wrong with

Since it's raining, wouldn't it be a good idea to bring an umbrella?

Look at a few other constructions to see how natural it is:

Since you're going to the store anyway, wouldn't you like to pick us up some ice cream?

Since our success is uncertain, wouldn't we be well advised to consider how to minimize our risks in case of a downturn?

The first clause assumes a fact, and the second balances it with a speculative counterpoint. That is grammatical, unexceptionable and, incidentally, one way to make a sentence interesting.