To add to the point above: Jerrygenn is totally right about the overheating issues. In my experience, trying to force a speedlight to keep up with an a9 II at high burst rates is just a recipe for a dead flash. Ive tried many setups over the years and honestly, the best way to save money and stay reliable is to be realistic about your recycle times.

• Look at the Godox V860III-S TTL Li-Ion Flash for Sony instead of the V1. You get almost the same performance for significantly less cash, and the Li-ion battery life is a lifesaver for long events.
• If you want to stay in the Sony ecosystem without breaking the bank, the Sony HVL-F46RM Wireless Radio Flash is a safer bet for longevity than the older models and it is much more consistent.
• Always keep your power settings as low as possible, like 1/32. No flash can fire at 1/1 for 20fps without melting. Basically, you dont need the flashiest gear to get the shot, just something that wont quit on you halfway through the game. Ive found that buying two mid-range units is always safer than one flagship if you are worried about gear failure in the field... just my two cents.

I have been tracking the flash market since the early DSLR days, and ngl, the way third-party tech has caught up is actually insane. My own journey involved moving away from overpriced OEM units to save my budget for high-speed glass, and honestly, the technical performance of mid-range units today is more than enough for the a9 II. From a market research perspective, you are paying a huge premium for the name brand when the internal capacitor technology is pretty much standardized now.

• Neewer Z1-S TTL Round Head Flash
• This is basically a budget beast that mimics the recycle speed of top-tier units for significantly less money.
• Westcott FJ80 II Speedlight
• I found the color consistency and thermal management here to be superior during long bursts compared to many entry-level units.
• Yongnuo YN685 II Sony
• A total workhorse if you want something that just works without the fancy round-head price tag. Tbh, unless you absolutely need the integrated weather sealing of the flagship Sony units, these budget options are the SMART move. I saved enough switching brands to pick up another prime lens, which is a massive win for sports work anyway. Basically, the price-to-performance ratio on the Neewer is CRAZY good right now.

Godox V1 Pro. https://www.bhphotovideo.com/c/search?Ntt=Godox+V1+Pro+sony&BI=8941&KBID=10361&SID=12345&DFF=d50

Late to the party but tbh before giving a final pick, what kind of distance are we talking about? Indoor volleyball is a totally different beast than shooting soccer from the sidelines, and that really dictates how much power you actually need. If you want a solid alternative that compares well to the Sony OEM units but at a way better price, check out the Westcott FJ80 II 80Ws Speedlight for Sony. It handles heat better than most and the touchscreen is actually super intuitive when youre in a rush. Quick tip: since you will definitely be using HSS on the a9 II for sports, just remember it eats your battery and slows recycle times. I usually bump my ISO a stop or two higher than I think I need just to help the flash keep up with the burst rate.

To add to the point above: I've been dealing with the exact same frustration on my a9 II lately. Honestly, it's so disappointing when the camera can handle insane bursts but the flash just dies after three shots. I've missed so many key moments because of recycle lag or overheating... it's just not as good as expected for the price we pay for this gear. I've had better luck going a bit DIY and skipping the traditional hotshoe mounting to get around the thermal limits. Here is what I am currently using to keep up with the frame rates:

• Godox AD100Pro Pocket Flash mounted on a bracket. It is small enough to stay mobile but has way more stamina than a standard speedlight.
• Godox PB960 Propac Battery Pack hooked up to a compatible flash. This is basically mandatory if you want those sub-second recycle times for fast action.
• Neewer Z1-S Round Head Flash for a budget option, though it still has some thermal issues if you're pushing it too hard. It is definitely a struggle to find the right balance but you'll get there. If you need help with the wiring or the bracket setup just let me know, happy to walk you through it.

For sports photography with the Sony a9 II, the Sony HVL-F60RM2 flash is highly recommended due to its fast recycle times and consistent output. High-speed sync (HSS) is necessary for shutter speeds beyond the a9 II's standard flash sync limit. To optimize flash settings, use HSS for faster shutter speeds and adjust flash power to balance exposure and recycle time.

You can try off camera flash, such as Godox AD200 Pro II.