my 2¢: if external storage is not an option for some reason, dropping ext4 in favor for F2FS will do a lot of good for sparing your sdcard from early death (due to superblock updates happening every so often), if not for performance due to ext4 not being flash-aware by design.

If that's not an option AND your setup has no power issues (e.g. it has good power supply, externally powered usb hub etc), disabling journalling might also improve performance: tune2fs -O ^has_journal /dev/sdaXXX (can't do it on mounted fs, so either do it from initram consolee, do pivot_root to minimal fs in ram, or do it from another machine)