I resized it down to 800Mb and that completed the first boot successfully

Sorry, but this is not how it works: if the system did not create the SHARE partition, your night was not successful, probably the problem is the quality of your SD card, I recommend testing with another one.
You should not change the size of the main partitions, the only partition you can shrink is the SHARE partition if you want to use dual boot with another system. the partitions are the necessary size, there is no "needlessly left", the space is "reserved for updates / temporary files / etc", there is a reason, it is not an error.