It’s not just you this is also my opinion, 5.3-codex was a fantastic model in terms of balancing output quality and cost.

Cheap and efficient enough I could afford to use it on basically everything unlike 5.5 or Opus, but still pretty good, I preferred it to sonnet

5.3 was incredibly better than 5.4/5.5. I stuck with it for months after 5.4 was released, and kept testing 5.4/5.5 every now and then but they both were too inconsistent, too rash. I switched to 5.5 a few weeks ago and now regret it, but I am no longer seeing 5.3 as an option to use, only 5.3-Spark, which is trash compared to 5.5.

They rendered 5.3 unusable for me a few weeks back. It simply was locking up or answering poorly.