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.
Yes, Apple iPhone 8 does have the VoLTE support.
iPhone 8 does support fast charging. Apple iPhone fast charges up to 50 percent in 30 minutes when you use an Apple USB-C to Lightning cable and one of these adapters: Apple 29W, 61W, or 87W USB-C Power Adapter.
Yes, iPhone 8 does have the OTG support.
No longer the gold standard, the iPhone 8 is still all the phone you need.