Interesting. I bought my Series 5 in 2020 and am still using it today. I've used Always-On-Display for the past four year too. No burn-in. So, as always, YMMV. Also, Series 5 OLED is single-stack OLED, not dual-stack.
This is just a reminder again that if you're looking in a relatively bright environment, it's much harder to see the difference between a mini-LED iPad Pro and an OLED iPad Pro. If it's really bright, like outside, then the OLED will be better because it can go brighter, but IMO, the real advantage is in very low light (eg. watching a video on the couch at night with the room lights dimmed). OLED blacks just look so much better than mini-LED, with no blooming. See the picture I posted above.
BTW, I was able to compare the 2024 OLED iPad Pro vs. the non-mini-LED LCD 2024 iPad Air. The 2024 iPad Air looks noticeably worse than the 2024 iPad Pro even in moderately bright room light. The contrast looks worse, and the viewing angles are slightly more narrow. The Air's screen is definitely a step down.