ext4 block size on eMMC

This is really a curiosity question: I’ve been messing about clearing up space on the eMMC after flashing a Debian image. After some modest housecleaning, I have about 384M available which just seems too little to me. Now I will be adding a microSD for storage but I was wondering if:

  1. changing the filesystem blocksize on the eMMC was possible. Most NAND flashes have 512 byte pages versus the ext4 block size of 4K. I noticed while poking around that the reported usage for files and directories was much higher than than the actual size of the files. This could be from many small files - sounds like Linux libraries to me. Can you run mkfs on a live ext4 disk or do I need to do it offline? Also, can you change the block size after the fact and keep your data?

  2. Is using bindings a good way to move directories off the eMMC and onto the uSD? What would be the best directories to move? /home for sure. /usr? /var? /opt?

Thanks in advance - learning this whole Linux thing and perusing the community information has been a great learning experience!