Drainage help back yard

Hi All,

Built a new home in 2022 the backyard had some drainage issues with the backyard flooding, I got a plumber out and they installed 3 storm water drains and plumbed it to the storm water. No more flooding issues but after rain the water sits in some areas around the alfresco and near the rear retaining wall, the backyard is only 10m x 2.5m, it does not get much sun as the rear boundary is a retaining wall and fence on top approx 3m high. I just received a quote rectify the issue, I was quoted $8000 for them to install a absorption pits around the existing storm water drains and connected them with a absorption trench. This seems excessive, has anyone encountered this issue?

Thank you for your responses