The one gotcha you'll run into with that approach is that if you have spillover enabled for your effects changing the output level could boost or cut the volume of your delay repeats and reverb tails suddenly. If you have your delay block set to MUTE OUT or THRU it won't be an issue because the moment you leave your delay scene all the delay gets cut off. But if you have MUTE IN set to let your delay repeats spillover into your rhythem preset after your solo the scene level could could abruptly change them.
You are right, this approach is about adjusting the amp level on multiple channels. It affects the amp level in the channel it's in, and it's changes will apply to all scenes using that channel. The scene level in the output would let you control scenes individually.
One of the hardest parts of learning the fractal system is how flexible it is: it allows you to do everything you could with a physical rig, including all the details of having different amps at different volumes and then different downstream levels to adjust them. So there are a number of different ways people can adjust the overall level of their scenes within a preset:
- Use amp channels, the same way that you would use the channel volumes on a multi-channel amp to level the volume of your different sounds.
- Use the output level of a downstream block, like the cab block or a volume block, as a separate way of boosting or cutting the volume of the block before it
- Using the scene level in the output block (with the caveat I meantioned above that effects splillover can behave badly using this approach).
I think most people use the amp block volume as their main volume setting because it best resembles a rig with a multi-channel amp people are used to. You set your clean volume in channel A, your rhythm volume in channel B, and your lead volume in channel C. Any other scenes like "lead but with a delay" just layer on top of the lead volume setting just like engaging a delay pedal would in the effects loop. Then you're only volume levelling a couple channels instead of 8 scenes.
But you could recreate this same footcontroller layout to control the output block level potentially. You'd have to look at the options available in the layout editor.