In terms of head loss, I believe that flex ends up worse than an elbow for head loss. Even two 45's is about the same as 1 90, they both add about the equivalent of 8-10 feet of pipe.
However, there are large radius 90's which can reduce head loss since they are smooth like normal 90's but have less of a transition.
One other option if you have a lot of room to make the bends is to heat up the pipe with a heat gun and put very long radius turns in the pipe itself. This has the lowest amount of head loss possible since you are only limited by how fast you need to make the turns.

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