We had priced out 18X36 everywhere just because it was considered standard. On a whim I asked how much the 20x40 was going to cost and to my GREAT suprise it was $500 more in concrete and everything else cost the same. We got a rectangle like yours and i imagine a curved liner would have been another extra charge but that $500 was the best $500 i ever spent.
Once you’ve recovered from this project financially, put in wider steps down to the pool. We did our deck and sliding glass door a few years after and went with 6ft stairs for an open unobstructed view from the house. It really makes a difference.