I would suggest being very careful about using that second slot for redundancy. Even a fast Sony SF-G Tough Series V90 SD 128GB will likely bottleneck your R3 buffer. You might want to consider these for the main slot:

• ProGrade Digital CFexpress Type B Cobalt 325GB: Excellent sustained write speeds.
• SanDisk Extreme Pro CFexpress Type B 512GB: Good value but might throttle... Make sure to prioritize sustained performance or you wont get those 30fps bursts consistently.

To add to the point above: i went through a similar headache when i first jumped into the r3 ecosystem. Honestly, im always a bit paranoid about data loss, but i also didnt have infinite cash left over after that body purchase. I ended up trying the Angelbird AV PRO CFexpress B SE 512GB and ive been super happy with how it performs. It isnt the most expensive card on the shelf by a long shot, but it hasnt failed me once during long bursts at the local track. The trick i found to save some money was to stop trying to force that redundancy in real-time. It just creates unnecessary heat and lag. I just shoot to the cfexpress for the heavy lifting and keep the sd slot for small jpegs or just keep it empty. Basically, it works well for me and i havent had a card fail yet, knock on wood... keeping things simple is usually safer anyway. No complaints here with that setup so far.

Unfortunately, you are probably gonna be disappointed if you try to use that SD slot for redundancy during 30fps bursts. It is a major bottleneck because the camera has to wait for the slowest card to finish writing, and V90 is just old tech compared to CFexpress. I have had issues with the buffer stalling for way too long because I tried to mirror files... it is honestly not as good as expected for a pro body. If you need performance on a budget, check these out:

• Delkin Devices Power CFexpress Type B 128GB handles sustained writes way better than SanDisk.
• Lexar Professional CFexpress Type B Diamond Series 256GB is technically the fastest card for PCIe 3.0 specs right now.
• Angelbird AV PRO CFexpress B SE 512GB is a solid middle ground for speed and cost. Tbh, stick to a single card for the track meet or youll miss the action while your camera is busy writing to that slow SD slot. It kinda sucks that the second slot is basically for backup only when you arent shooting fast.