What do you use it for?
If just email and light surfing without a keyboard and watching video, I wouldn't necessarily bother upgrading. That said, the iPad 8 is a huge upgrade over the iPad 5, esp. if you decide you want an external keyboard +/- Pencil.
IMO for general usage (excluding games and heavy content creation):
A9 + 2 GB RAM in iPad 5th gen is showing its age, but is still pretty usable.
A10 + 2 GB RAM in iPad 6th gen is also showing its age, but the SoC speed is better than iPad 5th gen.
A10 + 3 GB RAM in iPad 7th gen is very good. Very occasional short pauses can be noticed but it’s very decent overall. This is my wife’s daily driver and she’s more than satisfied with it.
A10X + 4 GB RAM in iPad Pro 2nd gen is excellent, a step up from iPad 7th generation although as mentioned that is already very good. iPad Pro 2nd gen is my primary machine.
A12X + 4 GB RAM in iPad Pro 3rd gen is excellent, and the speed advantage over A10X + 4 GB RAM is actually often hard to notice for such usage. Side by side, the iPad Pro 3rd gen wins of course, but the difference is surprisingly subtle.
iPad 8th generation is A12 + 3 GB RAM. I haven't used one myself but nonetheless it should be very good, with an SoC that is ballpark in the same league as A10X iPad Pro 2nd gen (sometimes better, sometimes worse), and the same amount of memory as iPad 7. So, I'm guessing the experience would be somewhere in between. Plus all the models from the iPad 7th gen and up are bigger than 10” and have the Smart Connector for keyboards.