here are my answers to few of your questions:
#3 - snapshot is daily update, while stable gets update less frequently
I have been using snapshots w my rpi4 and follow the daily updates for the none core pkgs, and do full image update once a while (bi-monthly)
#4 - Ext4 vs SquashFS
#5 - the way for upgrading is the same regardless of image, create backup, save pkg list and anything extra you did, upgrade and restore those extra pkgs, configs, users etc etc, (I scripted it)
#8 never required a serial console, think about it, you have ssh, if that fails just get the SD to an other system and correct issues
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