I spent way too much time diving into the thermal management specs and recycle consistency data before I settled on my current setup. For the a7R V, you really need something that can keep up with that high resolution sensor without overheating during a long event! I eventually moved away from the more budget options because I needed absolute reliability during back-to-back portrait sessions. Check these out for professional-grade performance:

• Profoto A10 AirTTL Sony provides incredible color consistency and a lightning-fast 1.0s recycle time at full power.
• Westcott FJ80 II Speedlight has a fantastic 1000+ full power shots per charge which is basically life-saving for long days. I recently did a three-hour gala shoot where I was firing almost constantly, and the Profoto was just amazing. It never throttled once! One thing to watch is the MI shoe connection... it can be a bit finicky on these newer Sony bodies so I always give the contacts a quick wipe before I start. Having that reliable TTL accuracy makes a massive difference when the lighting is changing fast!

Godox V1 Pro or Sony HVL-F60RM2.

Godox V1 or Godox AD200 Pro.

I've been using the Sony HVL-F60RM2 with my a7R V, and it's been fantastic. It's not cheap, but it's fully compatible with Sony's latest flash protocols and has excellent build quality. The recycle times are impressively fast, and the interface is intuitive. If you want a native Sony solution, this is the way to go.