Arduino Uno SD card storage
Regarding 1, it's true that EEPROM has limited write cycles. Mostly it can be much more, but it's only guaranteed upto that number, so I can imagine you don't want taking the risk to be lucky.

Regarding 2, other solutions could be external SRAM, which can be much more than just 1 KB, but also adds some extra components, thus cost and more important power. When that SRAM is full (can be many KBs), you can transfer it to the SD card.

Another (added) improvement you might think of (if you did not already) is data compression (trying to store the 50 bits in less bits), but this will cost some 'computing' power.