I've been in your shoes many times and honestly, I've tried basically every third-party brand out there over the years to keep my gear costs down. Shooting a wedding with just one battery is a recipe for a heart attack tho. You're right about the high-speed shutter stuff... most generic batteries wont trigger that green H plus icon on the R6 II because they dont have the right internal chips. In my experience, the only non-Canon brand that actually works consistently for the high frame rates without giving me a headache is SmallRig. I've been using their newest stuff for about a year now and it holds up.

• SmallRig LP-E6NH USB-C Rechargeable Camera Battery 4084
• SmallRig LP-E6NH Battery and Charger Kit 3821
• Wasabi Power LP-E6NH Lithium-Ion Battery 2-Pack The USB-C version is killer because you dont even need a dedicated charger... you just plug a phone cable straight into the battery itself. It's saved me during long drives between venues. I'd still suggest trying to scrape together the cash for just one official Canon LP-E6NH Lithium-Ion Battery specifically for the actual vows. Use the SmallRigs for the reception or getting ready shots. It gives you that peace of mind so you arent staring at the screen waiting for an error code while the couple is kissing. Whatever you do, skip the super cheap no-name brands on Amazon... they're okay for lights but I've had them swell up in my camera bodies before.

Building on the earlier suggestion, I have to disagree slightly about just grabbing any generic pack for a gig this important. For a high-stakes Vermont wedding, you really dont want to gamble with those $15 batteries that promise huge capacity but weigh nothing. The R6 Mark II is super picky and specifically checks for the LP-E6NH communication protocol to enable that 12fps mechanical shutter. If the camera doesnt like the voltage or the chip, your frame rate drops instantly and you will see a blinking icon instead of the green H+ you need for the walk down the aisle. I would suggest being very cautious and looking at these specific options:

• SmallRig LP-E6NH USB-C Rechargeable Camera Battery 2400mAh is a great choice because it has a built-in USB-C port. You can charge it in your car on the way to the venue without a bulky cradle.
• BM Premium LP-E6NH High Capacity Battery usually works without the non-genuine error message popping up on the screen.
• Watson LP-E6NH Li-Ion Battery Pack is what I usually suggest as the safest pro alternative to the official Canon ones. Whatever you do, make sure you test them the second they arrive. Fire off a long burst in mechanical shutter mode to see if that H+ stays green. If it turns white or starts flashing, that battery is basically a paperweight for weddings. Also, grabbing a Anker 737 Charger GaNPrime 120W might save your life if you need to fast charge the batteries during the dinner break. Just be careful with the really cheap stuff... weddings are one-shot deals.

Are you shooting much 4k video? Unfortunately, third-party cells are just disappointing.

• SmallRig LP-E6NH USB-C Battery: cheap, kills H+ fast
• Watson LP-E6NH Li-Ion Battery: stable, though pricier