Yes, you can play 4K videos on iPhone 8.
Yes, iPhone 8 does have the OTG support.
Yes, you can use the old AirPods with the new iPhone 8.
You will get about 51-52GB as free memory in case of iPhone 8 64GB. However, this is very much dependent on the software version.
No longer the gold standard, the iPhone 8 is still all the phone you need.