My last pool was shaped like that.
Go to the pool math page and below the main chart is an estimator.
I used that and came up with 23,000 gallons initially....then when I noted free chlorine level changes when
adding bleach...over about 2 months I nailed it down to 19,000 gallons.
Every time I added chlorine or other things like calcium, it turned out 19,000 is what it was.
My current pool is 13,000 gallons. Although the hand written paper work says 16,500.
But my estimates (same as i did before) came up at 13,000 according to pool math
and when I add chlorine the pool math is correct...so in my mind it is 13,000
A friend of mine has a pool just like yours looks (bigger than my old one but longer) and he claimed it was 30,000 so
you might be very close.