I prefer R1 or R10, though I certainly understand economics.

If you convert to r5, and that might work fine for your workload, I wouldn't add 2 drives, I'd add 3 and have a hot spare. If R5 fails, and you have multiple drives from the same vendor/batch/generation, it is entirely possible you lose another one in short order. I'd have a spare in there, alerts set up, and kick of a rebuild ASAP of the failed drive, which I'd then replace quickly.