Yes, iPhone 8 does have the OTG support.
Though Apple never mentions the battery capacity of their phone on their website this is what we have found during our battery tests of iPhone 7 Vs iPhone 8.
iPhone 7 has 1960 mAh Li-Ion battery while iPhone 8 has 1821 mAh Li-Ion battery. But thanks to the latest iOS 11 and the new Bionic chip support, iPhone 8 battery lasts longer.
Apart from the battery capacity difference, iPhone 8 also has wireless charging functionality and quick charging which are missing in iPhone 7.
Yes, iPhone 8 does support fast charging.
No. iPhone 8 does not have the FM radio.
No longer the gold standard, the iPhone 8 is still all the phone you need.