Simple question;
When using Attended Sysupgrade under Luci you have the possibility to store the generated firmware before flashing (Download firmware image).
Is there a similar functionality when using auc?
No, in auc it's all automated internally to download and install without pause.
To get the image, you can use https://firmware-selector.openwrt.org/ and add your extra packages there. Both auc and the firmware selector use the ASU build server on the backend, so the resulting img file is identical regardless of how you get there (LuCI Attended Sysupgrade uses ASU build server, too).
Don't know what the package list should be? The OpenWrt Firmware Selector - #621 by efahl